His mom and your breakup are not mutually exclusive. It is a fact that sometimes things happen to us in multiples and that's they way of life sometimes. That event is separate from you and your desire to end the relationship, and by unfortunate circumstance for him they may happen at the same time and that's his issue to cope with (or not cope as he seems to be doing). Are you willing to put your life on hold to wait for a different time? What if his dog dies, or he loses his job etc etc it could be you're waiting forever. If you are clear on your decision then the most honest and forthright thing to do is follow through - otherwise you run the risk of hurting yourself by continuing in a situation that you have already decided to discontinue, and the effect of that will pass on to him anyway.