He has definitly lost some words (ok, a lot), but he's done that with ALL his major-ish milestones. He became completely immobile for about a month after walking distances infrequently from 7-10 mos. Then he started crawling again for, oh, a whole day or so, and then was walking full time.
We do sign with him, and he's been pulling signs out of his head from a full year ago and using them here and there, and making up a lot of his own (and a kinda cool grammar... simple, but effective enough to make his c. 5 'regular' signs pretty useful, combined with a lot of one-offs.) And he says a lot of words, he just doesn't repeat any particular one more than about once a week or so. Exactly the same as he did with walking
Honestly, this sounds a lot like what DD did and from my understanding this is very common in toddlers. DD would say words a handful of time and then not repeat them. Eventually she just started talking and sure enough those words that I hadn't heard for months reappeared. My mom said I did the exact same thing as a toddler.
If you're worried about autism this doesn't sound the same. From my understanding with regressive autism is that kids know say 20 words use them regularly and then stop all of a sudden and can no longer communicate. What you're seeing is using words and then putting them on the back burner and using new ones instead. It sounds like he just putting them in the bank and learning new ones until he is ready to speak completely.