I know you posted this a while ago… but I've only recently joined here.
When I came out to my parents, my mum said the same things. She asked if it was just a phase and I felt absolutely crushed that she couldn't understand who I am.
2 years later, she still hasn't completely come to terms with my sexuality. She has only just started inviting my partner (who I have been with for 3 years) to family functions, rather than waiting for me to ask if she was invited (she would always have no problem with her being there, but still, the invitation is so much nicer).
Anyway my point is… It took me years to come to terms with my own sexuality… so I don't really expect that my mum will come to terms with it overnight either. The fact that she is trying is enough for me right now. And although it can sometimes get quite frustrating or upsetting that she doesn't automatically smile when I tell her how happy I am with my girlfriend or how we are at a point in our relationship where we want to start a family, I know that given time, she will always call me or text me or invite me over so that she can tell me that she loves me and my girlfriend.
From the sounds of it, you are trying and that is really important. It won't happen overnight, it won't happen in a few weeks… it may even take years like it has for my mum… but if you can show that you love your son and show that you are trying… I'm sure that he will begin to understand what you feeling like I have.