Is is me or he is just making excuses as to why he don't want to marry me or have a baby with me?

My BF & I have been together 6 yrs. He's got a 9 yo daughter and I have a 7 yo son. He's raised my son as his because his dad is MIA. We have a great relationship, besides he doesn't seem to want to marry me or have just 1 more child together. The marriage part I can handle but I want another baby so bad. He's a great dad to both our kids. His current excuse for not wanting a baby is (he's had many) that he's been raising a baby since he was 19 and he feels he doesn't want to start over, + he says he's too old to have more (he's 29) I told him that raising a child doesn't stop when their 18, it's a lifelong process, and we have 2 and 1 more would't hurt. We're financially stable and have an extra room in our home, + we have excellent family support. I don't want to leave him and have a baby with just anybody, I want one with him becasue he's so great of a person so that's out of the question. How do I get him to see my POV, or is it just me that turns him from this? PLEASE HELP.