Well from the picture, he doesn't necessarily look too underweight. Hard to tell with him being in shed if he's dehydrated but with your humidity numbers and being that he's eating,i wouldn't think so. He could just be a slow grower but you will know more as far as prey size once you get a scale and wether you should offer him more mice our just larger in general. Most recommend bumping them up to rats once they can take them as they say they offer higher nutritional numbers although I can't back that up myself. Speaking of your numbers, most recommend a basking/warm area around 90-92 and a cool side around 80-82 with ambient temps in the low to mid 80s.