Tummy time

My son will be 4 mo on the 20th, he hates being on his tummy. I prop him on the tummy time pillow or anything else I can find, put toys in front, help him grab them... he just drops his head and get mad. But he love sitting up vertical and laying in the swing to play with his feet. Do I need to make him stay on his belly longer even when mad a crying? Will his crawling be delayed from not having tummy time? How else did you get tummy time to work?