I have had my rat for a year and he is still afraid of me?

My rat is over a year old and I need advice on how to socialize him! I bought my rat when he was a baby at a pet store about a year ago (he is a feeder rat), and he is still very afraid of me. I admit that have not done the best job raising him. I should have gotten two rats and I should have worked harder to socialize him from the beginning. But he has always been terrified of me! When he does let me pick him up he freezes up, but usually when I try he screeches and tries to get away. I have tried tempting him with treats, I will put my hand in the cage with a spoon of hummus or apple sauce and wait for at least 5min until he eventually starts licking it. A little while ago I took him out (somehow) to clean his cage and I put him my rat proofed bathroom so he could run around. I gave him lettuce and he let me pick him up a few times! But since then he doesn't let me at all. I try to talk to him and pet him but he is just so frightened! It makes me so sad to see him like this still. I just don't know what to do! Please tips on how to socialize an older rat? (I hope this was clear enough I didn't read it over)