Maybe because the guitar is so prominent in your house, he dosent have the desire? For me as a kid it was such a mystical thing that I didn't understand, but I thought it was so cool. Until I got to the age where I understood it and had goals on it.
This was almost exactly my experience, actually.
My dad plays a bit of guitar and a lot of piano. His brother/my uncle plays a lot of guitar, and they were always really close so he'd visit a lot. I grew up in a house surrounded by music and musical instruments, and always thought music was "cool."
My dad tried to teach me piano as a kid, but it didn't stick. My uncle bought my brother and I half-scale guitars at about the same time, and we had a lot of fun banging around on them, but it was just making noise, never playing chords or actually learning anything.
What finally happened was I had to learn how to play guitar as part of a music class in 8th grade, right around the time grunge (which, lets be honest, is a pretty easy genre to start on, and that definitely helped) was getting big. I learned a few chords and got hooked, and it's been all downhill since then.
Just raising your son in a household where there are always instruments around and always music playing is absolutely the best thing you can do for now. It may not get ahold of him until later in life, but igniting an interest in music is still the most important thing you can do for him if you want to raise a musician.