He looks to be closer to a year old if not a year. Normally they taper off themselves. What are you currently feeding him? Depending on the bug depends on how much to feed. And frequency can be every other day.
So he looks to be about 12 inches... I am going with about 9 months old but genetics play into their size. Beautiful boy though. I think he looks really healthy no sign of weight issues with his casque. Stay with the every other day 5-7 feeders. Once he gets past the year mark then you want to start watching the weight more closely. Get a kitchen gram scale.