In the left pic you can see he has no fat stores in the bumps on top of his head-8 weeks later they are way fat :) lot of bananas and hand feeding until he was strong enough.
Is your ig pooping regularly? Or seems to eat ok but then just wont after a bite or 2. It is possible he might have a blockage.
your igs jaw is receeded but I have seen way worse and they seem to eat enough to maintain weight. If hes not pooping regularly Id visit a vet.