Can a 3some save a sex life? Is it worth it? Am I being clingy?

Hi guys,Basically my bf 33 and I 22 have been living together for 2 years, together for 3.There is a bit of an age gap... He did't come out gay till late while I was an early bloomer. I have slept with a lot of guys (Used to travel round the country with my family) and eventually got bored of hookups and past the "slaggy" phase to finally settle with my SO. I'm his second proper bf on the other hand and he regrets not sleeping around more in his youth. In a way he uses it as an excuse for the things he does.He cams with guys a lot, and uses whatsapp and kik to talk to a lot of guys. He went on a lads holiday a few years ago and cheated but I forgave, then early this year I found out he was going to meet a lad for sex which i found out prior to him going and I forgave him eventually (I had moved out and he begged and begged me to come back.. So I did) He also has a habit of kissing guys in clubs when he is drunk but blames it on the drink.. ReallyThing was going okay again.. Until he started spending more and more time on Kik and Skype. A girl friend of ours asked if we wanted a 3some, which I said obviously not, I'm monogamous. He laughed and said to me "Yeah we can find someone better than you for a 3some" I asked what he meant and he said there's loads of better guys to have a 3some with. I have already stated there is no way on God's green earth I want a 3some (Especially considering his history) He was so disappointed. He said I had one (I did when I was single with a couple. It didn't go well) He still goes on today about it would liven up our sex life and he sulks when I say no. He asks his wank buddies if they would have a 3some and then shoves it in my face when he says yes. He points at guys and ask if I would when we are out.. its madness. It does make me feel very insecure, the sight of him (and i have seen it in person) kissing another guy was heart breaking and now the thought of a 3some obviously would be worse!How do i get this idea out of his head? Is it benefiting to a relationship? Who do you even choose?TL;DR - Bf thinks 3some will save sex life, I didnt know there even was a problem with sex life. Do we go through with it and try it or stick to my guns?