Hi AnnaC and welcome.
Your DS sounds like a real little treasure! The way I've been trying to look at the gifted/non-gifted question with my DS3 and DD16mos is you basically can't wrong with assuming that your child *is* - if that means you will continue to provide him with stimulating new experiences and help him enjoy discovering the world we live in. There's absolutely no harm to be done in providing that kind of loving environment, although as you've probably discovered already, it can really be exhausting
In relation to the issue of words being 'lost', I have a slightly different take on this, care of my DD. Before she arrived on the scene, for whatever reason, I equated children being 'verbal' to being 'talkative'. I kind of figured that for a child to be talkative, as in using real words in context, then they would naturally require a large vocabulary to do this
and display it. The latter part is just not my DD, as she's the more quiet, introverted, "blink and you'll miss it" type. She started talking around 6mos with 'bubble bath' and 'wave bye bye' and quickly had over 100, so we stopped counting. However, by the time she turned 1, she became a lot more selective in what she said out loud. It scared us a little at the time, but I've come to realize that (with her at least) the words aren't 'lost', she just doesn't see the need to arbitrarily repeat the names of objects etc that she already knows. If she can look at DS3's (incomplete) drawing yesterday of the life-cycle of a butterfly and randomly say 'where's the cocoon gone?' and then walk off, I'm not concerned that she wouldn't say 'butterfly' (a previously well-used word) when I asked. Anyway, that's just our experience. Other poster's have made excellent points in terms of keeping a watchful eye on your DS's progress - better to be safe than sorry.
All the best with your little guy!
