ED? Or just not in to ME anymore?

Me and my soon to be husband have been together 4 years Now

Now a days when we try to have sex, or get ready to start, he will be hard as a rock but then when he penetrats with in sec's it goes away. This has happened the last 2 nights. The first night i was on top and as soon as i got up there it went down before i even had a chance. Last night he started it and it was going good till he entered then all gone again. I just don't know what i can do to help with this. We have tried viagra, but he still dosn't last that long, to me that crap don't work, he can't go 4 hours maybe 5-10 mins longer then normal. but he can't go after that iether.All this just started happening he's 33 could that have anything to do with it? Cause i'm 24. Don't know if age differance has anything to do with it. If maybe he hit that stage in life?