ok, what I am about to tell you is lessons learned from experience and heartache, so hear me out.
First, engaged at 16, well, nevermind.
Second, when you involve a sex organ in any way at all, you have gone too far. I know what you have been doing, and to be honest, you need to stop. You cannot be sexual with someone you are not married to. The way to bring it up to him it simply to say it. This may sound harsh, but you are young and really naive about all this. I thought I understood it all when I was 16, but I was horribly wrong. You are responsibly with honoring God with your body. Based on what you said, it sounds like in your heart you know your actions are wrong. Because you have responded to that, I am proud of you. So now that you have really thought about it, make the right decision, and keep things covered by underwear away from your boyfriend, and keep his area away from you.
I have found this very difficult to actually do. I am ready to be married, like last week. But I am not married yet. So recently my girlfriend and I prayed together and made a promise to God that we would not let wandering hands happen until our wedding day. Has it been tough? Yes. Has there been some mistakes? Yes. But I am happy that many boundaries have not been crossed, and I give God the credit for that. So if you respect his body, then you will not touch him sexually. If he respects your body, he will not touch you sexually. And if the two of you love Jesus Christ, then you will not touch each other sexually AT ALL until your wedding day. The first step is to talk to your boyfriend and clarify your boundaries. The step after that is to make a committment (both of you in prayer at the same time) to God, promising that you will keep your border. But I have to warn you, only make the promise if you are CERTAIN that you will keep it.
I hope some of this helps, respond to it and maybe we can talk in more detail about it. Trust me, this is worth figuring out. Stay with Christ always.