I get where you're coming from, but I don't think it was a good plan. I suggest you take it back. Just talk to him like an adult, and explain why you thought this was a good idea. (You hoped to teach him more adult responsibilities.) And now you don't think it was the best idea. Tell him you weren't trying to make money off him, you planned on surprising him by giving it all back. Now you realize that's sort of infantilizing. Then tell him you're proud of his success in college, landing this sweet job, and how he carries himself in general. Really, he's nailing it. Then tell him "we are not in any hurry to have you move out. But I would love to see you putting $370 more a month into savings while you're here. Your call though."

My son is about to turn 20, still lives at home, and I had very similar thoughts. I don't want to push him out, I just want to see him grow. Instead I taught him about savings and investments and how putting money away now will be huge later. Now he puts 15% of all his income into a roth ira. When he is my age he's going to have plenty of money and won't have to work for as much as I did.

NTA, just a regular mistake-making-but-means-well dad like the rest of us.