If they mix like oil and water, perhaps it's best that you don't try to force them to be all chummy. If they're content generally leaving each other to do their own things peacefully, why mess with that dynamic and make everyone miserable? And if they're actively antagonizing each other as is, it'd still be better to direct them to a truce and being civil to each other, rather than trying to make them best-buddies-forever when that will likely never happen.

Have you ever seen two jigsaw puzzle pieces that won't fit into each other because they are basically the same? That's your boyfriend and your son.

Sounds like you're forcing it just to make things convenient for you.

It'd just end badly for everyone if you tried forcing any kind of relationship between them.

Ironically, you succeeded in getting them unified. In how to keep you from meddling with their relationship, that is.