I have been with DP nearly 3 years. My DS is nearly 4 so when me and DP got together he was very young. Me and DP now have a 10 month old DD.
My DS adores DP and unfortunately does not have much of a relationship with his bio dad as his bio dad lives 100 miles away from us and only sees him twice a month, and is not really that great with him.
DP is really good with my DS and likes him, but I can tell that he doesn't "love" him. for example; It is clearly obvious when he comes in from work, his eyes light up when he sees our DD and he goes straight to her for cuddles but my DS kind of gets a "hello mate!" from him but nothing more loving than that. also, my DP seems to like it when its just me him and DD, ie, when DS is with his dad or when my parents have him.
me and DP are engaged and hoping to get married later in the year, and I really love him, we have a great relationship other than this, and as I say, he gets on with my DS but I think he should love him, not just be his "pal" - after all, he has been in his life as long as he can remember and is around a whole lot more than his real dad is.
I think I am rambling now...but some advice would be great.