According to surveys, men and wine have a 50% or higher chance to remain faithful in any given decade. But if you factor in the several decades, the odds of infidelity increase with age until his sex drive diminshes. However, the problem is actually women. A study was done, placing 2 "conservative" men in a "dating" relationships, one with a women who was "loose," the other with a "conservative" woman. Sex happened much quicker with the loose woman than the conservative one. Done in reverse, sex was delayed about the same time. This shows that it's the woman who controls sex, not men.
It depends. Is he a man of "means"? Then that could "mean" he has a hot little dish holed up in some apartment somewhere. Is he a politician? Not a good sign. But yeah I think many men in their 50's could stay committed. If you really want commitment you may try one committed to a penal institution--They aint going anywhere, at least for awhile, and I believe many are able to have conjugal visits : D