it shouldn't be bad for him, budgies live in flocks in the wild so it might even help him feel like there's other budgies around! i also play the radio or an audiobook for my birds when i'm not home, they sing and whistle at it haha so i guess they like it as long as you interact with him plenty he shouldn't feel lonely, ofc it's better to have more than one since they're flock animals, but budgies like people so it's not bad to keep one alone if you give him plenty of interesting toys and attention every day! : D
you can make fun bird toys out of trash! like toilet rolls, straws, paper, stuff like that :0 pets.thenest.com/homemade-budg… this is just one site i found, there's tons more if you just look up diy bird toys! my birds love cut up pieces of old shirts tied on a hanger, kinda like this i.pinimg.com/736x/c0/a4/54/c0a… they can sit on it and play with the pieces of fabric! foraging toys are also great, it keeps them busy and entertained! you can put food and paper or tissue into a toilet paper roll (you might have to show him how this works or just put food inside until he learns how to play with it)
and it takes a long time to gain their trust, especially if you got him from a pet store he might have been mishandled & scared of hands www.wikihow.com/Tame-Your-Budg… those are the steps to follow, and it might take A LOT longer than you'd like or expect i got one of my cockatiels like three-ish years ago? and he's just now allowing me to get him on my hand but that's a ton of progress for him! he used to be absolutely petrified of me ;w; leafical also knows a lot about birds, if you note them they'd be happy to give you some bird advice if you need it!
and i'm sure you haven't made his life worse ;o; he's in a safe home now, he's definitely happy about that! and if you keep working with him he'll learn to trust and love you! c: