Can one say:
1-I thought of him as playing football.
I thought of him playing football.
To me, the two have slightly different meanings. In the second sentence you are saying that you thought about a person, and that in your imagination when you were thinking about him, he was actually playing football.
The first suggests that you think that the person you were thinking about is the kind of person that you CAN imagine playing football.
My son really likes playing tennis.
Really? I always thought of him as playing football.
The response isn't very good English. It would be better said "I always thought of him as they type who would play football".