Why does my husband say no?

Since day one I feel like I always wanted more sex than him, I’m 24 and he’s 34. We’ve been together for 4 years now and 1 year old toddler. The first year of dating we would do it all the time, we moved in together it slowed down a bit. And really slowed down when I had our child. I’m beyond confused. He will slap my butt and grab my boobs, tells me all the time I’m sexy and beautiful. BUT… I’ll initiate sex even quickies just to get something. He always has excuses: I’m tired, I have a headache, I was gonna play my video game, and my favorite “I’m not as young I can’t keep up”. However, 9/10 times if he asks for sex, we will do it. I feel insecure when he turns me down. He tells me he’s attractive to me and so forth but I pick the wrong times? I would think we’re all home and our child is sleeping in the other room is a good time to have sex. The rejection is making me a certain way and I can’t understand where he’s coming from.