Is he gone forever? :(

No. He'll return eventually, but will be changed into a Riftborn pet. I threw one in yesterday and he returned after a few hours. When the feature first launched, it took a couple of days, IIRC. I think that was just for the launch, and it doesn't take that long now. Don't know if time can vary though.

If he's a species than can be turned Riftborn, then he will just change colour, as seen here.

If his species is not yet available in Riftborn, he will change into one of the two Rift pets.

oh! ok phew, ty