I wouldn't worry too much just yet.
Keep up with the tummy time, but when he gets frusterated its ok. As he gets older he will handle it better. Eventually he will be interested in toys & will roll onto his tummy.
My DS is 5 months & gets sick of it after a while.
It's not so much the crawling you need to worry about. I think tummy time is more to strengthen their back muscles for sitting.
He's only little. A few minutes at a time is ok. Gradually get longer as he gets stronger in the neck & back.