'Since' and 'From'##'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'##'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.