Together with my best friend Jon, I'm in a group of people who enjoy RPGs. I've been GMing for several years in this larger group. Other friends of that group also GM. I'm the most story-oriented GM, the other GMs are mostly one-shots and Door-Monster-Treasure oriented.
Recently, we started a new campaign. I'm GMing and there are 5 other players, including Jon, for the first time in a group with me. The campaign, I said from the start, even when only pitching it, would be very story-oriented and there would not be many combats. So every player made their character focusing on that. Jon doesn't read much English so we built his character together. His pitch for the character was that he likes to hit and we made a very fight-oriented character. That's OK, even though I reminded him that there wouldn't be much combat.
Now in session 1 and 2, there were combat moments, so Jon was happy. But session 3 came and it was mostly a downtime session. Jon was disappointed that there wasn't much combat and made his character look for illegal fights to make some bucks. Fine, so we did that, and it was great as I surprised the players with some tricks.
Now, I saw Jon during that session 3 and basically he didn't speak much. He let the story flow and was following the story, actively, reacted to it outside-the-game, but did nothing much in-game except following other characters. Other players weren't really aware that Jon's character was mostly passive except for the illegal fight as they were having real fun and they didn't see the time fly.
I've spoken with Jon and he told me that he was a bit sad there wasn't more combat, but he really enjoyed the story unfolding before him, and he excitedly looks forward to the next session, even though I told him it'd be like this session 3 where I plan no combat.
Separately, Jon really wanted me to introduce his GF to D&D, so last week I GM'd a one-shot for Jon, his GF and his GF's sister (who incidentally also loves D&D, but is not in my usual groups). For that one-shot, I used Candlekeep Mysteries' level 1 story but removed a few monsters because there were too many for only 3 players. And, again, Jon was there to follow the story, genuinely enjoying it outside-the-game, but the two in-game drivers were his GF (very good player for a first-time player) and her sister. Jon's character did nothing but the fighting bits or providing help when needed.
I know that the important thing is that everybody has fun in the games, but I want to make sure that these sessions appeal to everyone, including Jon.
I will definitely speak with Jon more about this topic, but I'm wondering if I should completely change my view on my campaign, making it more combat-oriented than I intended despite the initial expectation that it'd be a story-based campaign? If so, how should I handle this? Also, will Jon be a problem-player? If yes, how should I handle that part?
At present, it sounds like everyone (including your 'non-RP' player) is having fun. So, as of now, there isn't a problem here. But, you've said in the comments that you mostly want to know if this may become a problem, and how to handle it early...
Just keep checking in, make sure he's having fun. I have DM'd for players like this in the past and had zero problems with them--but it's important to talk to your players and make sure they are still enjoying themselves. And, just to be clear here...I would advise against asking questions that encourage a reflexive yes.
For example: "Are you having fun in this campaign?" is a question that encourages a reflexive yes. Some players will agree to questions like this even if they aren't really having as much fun as they wish they were--and they'll do it for any number of reasons ranging from a non-confrontational nature, to not wanting to make you feel bad, to not wanting to upset their SO who is enjoying the game, to any number of other reasons.
A better sort of question would be: "Is there anything about the campaign you wish was different?" or "I was wondering if I could get some feedback from you on how you think the game is going." Keep your questions open-ended.
Oh, and don't just ask him...make sure he doesn't feel singled out in this way. Honestly, checking in with all of your players like this is a pretty good idea in my experience.
Actual Warning Signs
Disengaged players are a potential problem...so here are some red flags to watch out for.
Dealing with these
Again, communication is key. It's important to make sure that you're all happy with playing the same game. The Same Page Tool is a pretty commonly recommended questionnaire used by DMs to make sure that everyone is on board with the sort of game you're playing.
Another possibility here is to conduct a Session Zero--which is simply a time when you get the group together and talk about what sort of game everyone is looking to play, and set everyone's expectations. And no, Session Zero doesn't have to be the first session you have--you can always have one later if you find you need it. (Personally, I have usually handled 'Session Zero' topics via email, google-doc surveys, and text-chat)
After all, if 4 players show up expecting to play Skyrim and one player shows up expecting to play Dark Souls...somebody isn't going to be playing the game they expected, and that can cause problems.
So, as in most cases where interpersonal issues can potentially disrupt gaming...you just have to communicate with each other. Be clear about what everyone wants out of the game, see if you can settle on something everyone is happy with (or conclude that this isn't the game for someone--which is fine too).