You hardly know enough about this man, to form an objective and realistic opinion.
Here's a line for ya......"one story's good, til another's told" meaning simply, his version of events is precisely that, HIS version. His ex will no doubt have her own "take" on their relationship.
I was once in a relationship with a man who had no kids. Turned out SIX months down the line, his "niece" was in fact his daughter; his ex-wife "trapped" him into marrying her because she was pregnant. NO! In a roundabout kind of way, I was "acquainted" with this evil ex and she didn't trap him. He happily walked up the aisle and played happy families until.....he was unfaithful to her.
If, as time goes by, your relationship is sound and the ex really is that heinous (and she WILL HAVE HER REASONS, BELIEVE ME) then, by all means, ask for advice and I'm sure you'll get it. In the meantime, step back, be a peripheral part of this father/son relationship (in case you're a part of their future) and see this for what (at the moment it is). You've met a lovely man, you love being with him....he may be your "one". In time. Enjoy that. Don't get bogged down in their "ex" relationship.
And for God's sake, ignore JeremyVile's advice in insisting this man cuts off his young son. That road leads to disaster for his son, any relationship you may all have in future and the boy's adulthood. This little boy didn't ask for all this stuff!