Cis lesbian here. I mean this gently- does it bother you that your son might be gay/bi/pan? I suppose what I'm wondering is, do you need this information? Sexuality is so complex, and your son is young- as I assume the young man he's dating is too. Is it reassurance for yourself you're after? If so, I personally believe there's nothing wrong with that- as parents we often need some context to understand our kids, especially with the world so different to how it was. You've not said anything that indicates that he's in any way struggling though?
In terms of any ways you might want to support them, I think the starting point for me would be, this is the relationship he's in, it's a same-gender relationship and so he will face some forms of societal prejudice as a result. Lots of things which might seem quite minor individually but which do add up to become irritating, annoying, difficult, but also actually sometimes extremely upsetting and dangerous- although that's improved hugely with legal recognition of our relationships.
Unfortunately, as a young trans man, his partner will face some prejudice, and your son might also. But I think- and I'm basing this on my experience as a lesbian growing up in the 1980s, section 28- that you can do a huge amount by listening to them, supporting them, and just accepting them.
Certainly I, as a cis lesbian married to another cis lesbian, and with kids around your son's age, would absolutely see this as a same-gender relationship, and I'd absolutely regard him as gay, bi, pan or however he identified based upon that relationship, assuming that is how he identifies.
I obviously have very little information aside from your first post, but he sounds like a lovely lad and you're clearly doing something right :-)