I'm not an expert yet but to throw in my two cents... if it's a 20 gal with only the items you mentioned, that seems awfully big and sparse for a baby; it may be making him feel exposed and insecure. What hides are you using; do they cover him completely? Do you have some extra decor in there to cramp it up and give him extra places to hide? My smaller snake came in a 29 gallon but I made sure he had plenty of things to clutter it up and help him feel less exposed. And I wouldn't recommend the more exposed half log hides like I have; my snake has no problem using them but many do.
I use the JumpStart thermostat and I like it a lot. Probably don't take advice from the reptile shopkeeper anymore. :/
Sounds like you're on the right track, though!
Oh gosh, wait, one more thing! If he's trying to escape and you have a lid that pops open... well, he can. And if he probably can, he probably will. Happened to me. Might want to pick up some lid clips or lid locks...