Do you have a thermometer (to read temps) for both ends of the tank? You will need a way to monitor both the cool end temp and the "hot spot" over the UTH. A infrared temp gun comes in handy for this, if you can get one (sometimes hardware stores sell them or you can buy online).
I use a thermometer like
this one and it will read the hotspot temp with a probe and if you put the unit itself on the cool end, it reads that temp too (as well as humidity).
What sort of heat source are you using (bulbs, UTH,)? When I kept my snakes in glass tanks, I had to use a UTH (hooked up to a thermostat) for the hotspot and a heat lamp (red bulb) to raise the ambient temps... Glass tanks aren't well insulated so the heat bulb was needed to keep the air warm enough. If you aren't using a UTH, I recommend
these. They don't get super hot, last a long time, and work great when hooked up to a thermostat. I have used
this thermostat before and it works well and I've also used a
herpstat, which is great if you want to spend the extra money.
He should be fine in a 3 foot tank for now, but boas generally get 5 - 6 feet or more as adults, so have plans for a larger cage down the road (4' x 2' x 18" high or similar).
Also, one other bit of info. With boas, you want to feed a food item that won't leave a "lump" in them and you also feed less often than with ball pythons. I feed my 3.5 foot male once every 2 weeks, but he is on small rats. You feed young boas (under one year I believe) once a week. You will want to see that the boa keeps a "loaf" shape, like a loaf of bread. If he starts to get round (like a ball python) he is too fat.
Good luck with your little guy!
EDIT: I also can't recommend
this book enough. It'll tell you everything you'd want to know about boas and more. It has great pics too. Much superior to other care books on the market.
