Humm yes 7 inches in a year is a
little on the small side. But he does appear to still have quite a juvenile look to him, over sized mouth and eyes, so he could just be waiting for a growth spurt. Bloodworms are fine but are not going to provide the bulk protein he needs to really chunk up. When mine was his size I had her on live bloodworms, mussels, crill, prawn, live fish, live crabs and live ghost shrimp, and shell on cockles and clams for his teeth. Maybe also try live morio worms once a week (high protein and fat content), mine hit 10-12 inches in the first year, but she was a bit spoilt! As he is on his own (pretty much) in the tank, so water quality should be good... so maybe worth increasing his feeding and variation? Just an idea. Try the local supermarket for frozen seafood mix and frozen fish pie mix (normally salmon, cod etc) which is quite a cheap way to bulk feed and vary the diet.