7 month old still not on solids?

I started offering my son solids a week after he turned 6 months. I mashed up a ripe avocado and he took 3 small bites from it and was satisfied & so was I. I tried it again the next day and he showed no interest so I didn't push it. A couple days later we tried a mashed up banana and he took a couple of bites and was done. So I did the same thing, tried it the next day and once again he showed no interest. He is now 7 months 1 wk old and pretty much refuses any kind of fruit or veggie I offer him. He is gaining weight, has plenty of poo & pee diapers and is basically only on his formula. I try about every other night to get him to take an interest in either a veggie or a fruit but he'd rather play with the spoon and have fun make messes. So my question is, is this abnormal for an infant his age to not be on a proper diet of solids yet? I have not called my pediatrician because I'm not really worried considering he's as healthy as can be and consumes approx. 36oz. of formula a day. So anyone gone through a similar situation? Any adivce or corrective criticism is welcome.