It is odd and a little bit off-putting that he's had more connection with Wanda over the recent years, thanks to stuff like Children's Crusade, than he seems to have with his *actual parents* who, AFAIK, have done nothing worthy of being forgotten (other than probably only being alive because of this, since living family tend to be endangered and / or snuffed off as a source of drama / angst, when certain writers remember them...).
Maybe Billy 'cast a spell' that makes everyone forget the Kaplans are his parents. For their own protection!
"Let's hurt that little punk by going after his parents!" "Yeah, that witch-y Avenger chick? No thanks. Don't want to be turned into a toad..."
Who knows if Tommy even
has parents. His entire backstory can be summed up as, "Busted out of juvie. Met Coat of Arms there." and it took several years to get even that second sentence.
It would be dark irony if he didn't have parents, because (dun-dun!), it wasn't Hulkling that was subconciously reality-warped into existence by Billy to be the 'perfect boyfriend' (as Kid Loki implied to sow seeds of doubt in them) but Tommy, to be the 'brother I never had.'