Does it have a hotspot? An ambient temp of 80 should be alright if you have a hotspot for him as well.

A three week feeding schedule is normal with these.

My tablet hates Photobucket so I had to look at the pics on my phone. From what I can see he looks pretty normal. These are long and lean snakes and being able to see their muscle and such is common, especially in a young snake that has not filled out completely.