Hi Tyler, you're doing a great job reaching out for help and gotten some good advice. It's nice that your grandma wants to help ! Definitely take the undertank heater back though. Your # 1 pproblem will be the lighting. Babies can not thrive without uvb no matter what type of vitamins + calcium you give them. Taking him out in the sun [ NEVER in a glass tank though ! ] is the best thing for him until you get the much needed
uvb bulb that was mentioned. One other thing to keep in mind is that some babies come from sickly breeder [ mother ] dragons so that can affect their health, so can bad care at a pet store. If he was from a store, was he + the other dragons healthy + alert looking ?
For now I would set him up in a smaller enclosure, get a plastic tote like I have in this link, it's how I raised all my babies that I bred years ago.
https://www.beardeddragon.org/media/29889/full
That way all the lights will be closer + he'll get a little bit of uvb from the weak bulb you have.