Could you post a picture of him? Now that it has been mentioned it does seem like he could be over weight. My year and half old boa is about 3ft and only about 500-600g (I haven't weighed her in a while). Boas aren't supposed to be round like pythons. A healthy boa will have a box like shape to it and unlike ball pythons boas will eat anything at anytime.
My boa for example:
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