Learn English – Is it a “visit by someone” or “from someone” when they visit

Using on here is perfectly idiomatic. And it's quoted as an example in a respected dictionary:

noun
(congratulations)
words expressing one’s praise for an achievement or good wishes on a special occasion:
our congratulations to the winners

[as exclamation]:
congratulations on a job well done!

• [mass noun] the action of expressing congratulations:
  he began pumping the hand of his son in congratulation

Congratulations for would be understood. Google Ngrams shows that on is far more common than for, though.

[As an aside, in TCS sounds odd, but I've no idea what TCS is, so it may be fine.]

'Since' and 'From'

'Since' talks about one particular point of time till now.

I have been working since 7 am

'From' in most of the cases talks about the starting point and ending point. Said that, the format could be 'from....to/till/until'

The shop is open from 9 in the morning to 6 in the evening

Quite close is 'since' and 'for'

'Since' and 'For'

Okay, as I said, 'since' talks about a particular point. So, you need to mention "since" which date the thing has been happening.

On the other hand, 'for' is used for a total period of time and you don't need to define a starting point.

The best example I can think of is...

I have been waiting for two hours

but...

I have been waiting since 2 o'clock

In the latter example, you need to define the starting point.

Another example:

I have been living in this house for 10 years
I have been living in this house since 2005

So, in your case, you have known him for the past 15 years OR since 2000.