Most Helpful Guy
It's going to hurt like hell and he is not going to want to be your 'first reserve' or fall back position. He may put on a brave face but there is no easy way other than to break with him because you don't think your relationship with him is good enough (at this time of your lives). Then wait a few weeks before seeing the other guy.
This way, if the second guy is not what he seems and it does not work you could in maybe six months have a possibility of re-evaluating your first relationship with a view to making your first relationship good enough. Address what the problems were directly rather than accepting them as unchangeable and needing to look at someone else..