It's a very skinny gibby
I'm sure he'll be better in no time though
It's possible they can reach 24" apparently, but the norm is about 18", but TBH if he's been badly cared for in the past - it's unlikely he'll get that big. I'd say he'll reach 15" or so, but who knows - it's always best to prepare for the worst...erm...if you get me. You need a 5 x 2 x 2ft tank for a gibby really, obviously the smaller they are the less you could get away with - but the bigger the better as he may have a growth spurt now he's being cared for properly.
Gibbies are omnis so veggies, good wafers, shrimp/prawns once a week, catfish pellets now and again...that kind of thing.
And Hawkins wasn't actually that huge for a gibby, he was only 15" - had to rehome him as our tank was too small for him to move about it (6ft x 2ft x 18"). It was the 18" bit that did it - had we had the 6 x 2 x 2ft at that point, we'd have kept him