Im going to take a guess you’ve got a Fahaka puffer?
Now, as for growth rate that is just an average and everyones experiences will differ depending on tank size, water quality, food, parasites and even the genes of said fish and how they were kept prior to being bought...
With that said, how long has this tank been setup, how did you set it up and have you tested your parameters recently?
My other advise might be to consider finding a more varied diet for your puffer as blood worms are mainly just water and dont carry much nutrition, you can try other worms such as live black worms kept in a container inside the tank so they dont end up everywhere, pieces of fresh seafood cut down to smaller size and even just chuck in a few more snails and let your puffer goto town on them...
Also, since your puffer is young you can pretty much feed him every day till his belly is rounded but not overly stuffed, then as he puts some size on him you can cut down the feedings slowly to every other day or even every few days as he grows...
If after time he still doesnt seem to be growing then that could mean your puffer is stunted from potentially being kept in a tank with bad water quality / that was too small for too long prior to you buying him or even having some other underlying issue such as genetics or even maybe IP’s...
If i remember correctly there was one member here before that said he had a Fahaka that refused to grow past 1-2” as well although what eventually happened to that puffer im not sure?