Can I sign into the PSN account on multiple devices

According to the PlayStation FAQ:

You can only activate one PS4™ as your ‘Primary PS4’.

Your primary PS4 has unrestricted access to all content on all accounts. But a non-primary PS4 can only access the content while logged in.

Here's what you can do: set up your son's PS4 as your primary PS4. That way, he will be able to use the Plus features while logged on his account. The downside is that on your PS4, you have to log into your PSN account to do anything, that means you'll be blocked if the Internet is out.

This is entirely different on the PS3, as it allows you to activate two unrestricted consoles.

According to this site, you are able to do what you described for you and your friend to share games. However, in order to play your own games, you will be required to have an active connection to the internet at all times, which is less than ideal.

However, because your own PS4 will not be registered as your primary PS4, no one besides you can access your games on your PS4. However, everyone who has access to your console will be able to play your friend's games, since that would be the account with your PS4 listed as its primary. Likewise, anyone that plays on your friend's PS4 could access games that you have purchased on your account.

Savefiles are kept on a per-account basis, so any games that your brother plays on his account will not affect you or your friend's savefiles in any way.