Advice for tips with boyfriend

So me and my boyfriend have been staring to become more sexual, working our way up towards sex. The other night we were making out and I started to give him a hand job. I have never done anything with a boy before, he is my first boyfriend and I am his first girlfriend. So we are both pretty new to this. We are 17 so this is a good age i feel to start experimenting. The only problem is, since I am inexperienced I don’t want to make it to where he isn’t enjoying it. So far it has been going great! The other night when I gave him a hand job I got him to finish, which was good. Then the next day we tried again and I ended up hurting him a little, he told me right away and I tried to fix what I was doing but I wasn’t able to get him to finish. Tonight the same thing happened and I hurt him a little bit. He told me not to worry because I’m new to this and we will learn together, but I don’t want it to not be good for him. He told me so far im doing really good and that he likes what I’m doing, but I was only able to get him to finish that one time. These past two times I don’t know what I did different but I couldn’t get him to. He said he was close a few times but then I had moved. What can I do to be better at this? Do you guys have any tips?