Denny is really cute. I personally like it a lot. And there seem to be plenty of young Lenny’s about now, so he’d fit right in with his peers (although I adored Jeffrey Dean Morgan’s Denny Duquette on Greys Anatomy years ago so my love for the name may be influenced by that 🤣)
However I’m not sure whether every adult man would appreciate having a “cute” name or not - so I’d usually suggest something longer on the BC to give him options. But I think Dennis is awful, sorry - Dennis the Menace and Dennis Pennis jokes and just doesn’t have a very nice sound (a bit like cute Archie vs Archibald).
Maybe Desmond could be an option? It’s not at all a favourite but imo it’s better than Dennis and you could still use the nn Denny (as long you used it from day 1 otherwise you risk “Des”).
Having said all that though, there are so many boys in the current generation with nickname style “ie/y” ending names, that by the time Denny grows up they probably won’t sound so cutesy anymore.
ETA: I love the suggestion of Daniel, it’s a timeless “can’t go wrong” classic. And the “Den” letters are there for Denny. Like with Desmond though, you’d have to use the nickname from Day 1, otherwise it’d automatically get shortened to the more common Dan/Danny once he’s older imo.