Is it necessary to tell him?

No break up is easy. It's completely up to you whether you tell him - but either way you have to make yourself clear, and then cut contact to make it easier for him. The truth will hurt him - he may not accept it for a long time, but he will eventually move on too. Telling him about this guy could go either way - it may be the wake-up-call he needs, or it may just depress him further. Sometimes you have to be cruel to be kind, especially if someone just doesn't seem to get the message - but only be cruel as a last resort. If there is another way, then do all you can to make him understand that it's over, before you deal him that kind of blow. If you haven't talked much about the breakup and there are unresolved issues, then it is a good idea to see him and discuss what went wrong. Answer his questions, be upfront, go over it as many times as necessary, but then after that one meeting, don't keep going over things. Once you've explained it's over, that you're happy single, that you have no intention of getting back together, you shouldn't keep going over it. He'll need time to get his head around the fact, so don't respond to his calls or texts, and don't contact him yourself. Sounds drastic, but it's the kindest way in the end. You may have moved on and feel ready to just be friends, but to expect friendship from him when he still isn't over you is a little selfish. Keeping him in your life on your terms probably isn't the fairest thing on him. It's clear you've moved on and you're happy with the single life, so don't feel guilty about this other guy - your relationship had ended and it was the closure you needed.