M.P.
It is supposed to be 10-15 minutes per grade. My 6 year old gets basically NO homework. Maybe a worksheet once in a while and is supposed to read or be read to every night for 10-15 minutes. I would caution you on giving him mom homework. Sounds like you are not doing "that" much but try to make it as fun as you can for him. Busy work like repetitive writing will zap a kid's love of learning faster than anything else. And in just a few short years he will be spending his whole night doing homework.
Also-there are some really great websites out there that you could go to with him where he can learn but have fun while doing it.