This has Cannondale written all over it. Tall guy, big guy.
I’d find any Cannondale frame he takes a liking to, the ST Tourers that I like came in big size jumps. I use a 23, I’d suggest a 25 for your son, if he likes the SR Sports frames, they’re more varied, I think (but not sure) they were sized in 2cm increments.
They have vertical drop outs, which is good, because no wheel slip,
and is bad, because chain tension
. You can find eccentric hubs, and more expensive, eccentric bottom brackets. That will get your the chain tensioning adjustability you need.
I have a couple Cannonballs, love how they don’t flex. After they were welded the whole frame was heat treated as a piece, a true monocoque frame build. The strongest frames were made between ‘85 and about ‘91/92, way overbuilt.