Is he at a good size for his age?

Thank you Gio

Boas will constantly grow throughout their lives, just at a much slower rate after about 3 years old. So yes he will get bigger but it will be slow, nothing you would probably even notice. Also it depends on the morph of the snake concerning its "chunkiness". I was told some hypos get thicker than the usual breadloaf shape but are still healthy. Mostl I would just watch to make sure your boa has the lateral muscle line going down the side for a lack of a better term. If he starts to look like a cookie dough tube and all squishy, then hes getting fat. Rosey is a perfect example, she seems rather chunky but when she is out and crawling around, she has muscle tone down her sides and top and can squeeze pretty darn good(she does this to my arm when she doesn't want to go back in the cage) yet if you set her down on the bed or something soft and flat, she looks kind of chunky. Where as Vicky is the cookie cutter boa profile of thin but tall with the same muscle tone as Rosey. Rosey is 3 years old and 6' and Vicky is 1 year old and 3'. Another tell tale sign of fat is if their head seems abnormally small for their body, like a little pin head on a huge body. Then you know it was powerfed.

From what I have read, most of the significant growth of Boas is accomplished within the first four years. Of course there are always exceptions.

Like others have said, boas grow at variable rates throughout their life. Ive had boas that were seemingly going to be on the small end, and put on a ton of weight and size randomly to become my biggest. Just never know what youre going to get. However, a hot and well fed boa will grow large quickly.