I don’t think you can say that it is or isn’t an eating disorder. What you are seeing as fluctuations in appetite, could be because he’s been restricting his eating (deliberately or not) and then has massive hunger cues and catching up. Or at least catching up a bit.
My 18 yo dd does this. She isn’t consciously aware of her restricting. She has ADHD and is chaotic, won’t have a routine right now, sleep all over the place, and she literally forgets to eat. And because she has autism, she doesn’t get hunger cues until she’s starving.
Are you able to discuss with your ds why he isn’t able to eat those times? And about the difficulty swallowing?
You say he is pretty adventurous with food, yet right now he isn’t. If you were to cook him one a proper meal, as opposed to leaving him to it, would he eat it? And if he would, will he eat cooked meals every day?
It sounds as if your ds’s diet has reduced to very beige foods. There is pretty much no protein in there and no meat if he previously ate that. Or any plant based alternative beyond a bit of peanut butter if not. I’d also be very concerned with that diet. Restricting to a small range of ‘safe’ foods sounds a lot like ARFID. ARFID, which has potentially tipped into anoretic behaviours. Difficulty swallowing / gagging is also a sign of ARFID.
From everything you’ve said, I’d be very concerned. As @likelysuspect said, some people don’t stop eating altogether. Some never restrict to fewer than 1000 calories. I would therefore be looking to getting him to a GP ASAP for basic observations to check his BP and heart rate, which will give an indication of just how much this is affecting him and a referral to adult services. If you have a cuff, I would do this yourself. He needs to be still for at least 10 minutes to get an accurate reading.
It’s important to ascertain stuff like - How much is he losing in a week? How much has he lost? How many calories is he eating in a day? I would be counting his calories every day right now. Ie monitoring the cupboards and seeing what’s missing. It’s a PIA to do this and weigh everything.
Everything you’re describing is exactly what my dd did and she was very ill. Didn’t believe she needed to eat to live in the end.
As @Girliefriendlikespuppies said, the standard is 3 meals and 3 snacks every 2 to 3 hours to stabilise blood sugars and to get the body used to eating again. As an adult, he will need to want to do that for himself. And whilst he’s living with you, he may let you help him with it.