Think of it this way- tummy time is especially important for his muscle development. Much like brushing teeth is important for health teeth. It's not always fun,no, but by giving it to him in small "doses", you will be able to increase his tolerance for it. 3-5 mins at a time is typical of his age. Just try to do it a few times a day, and "save" him before he really gets angry and frustrated. Make it fun. Try laying on your back and laying him on your chest for some fun "face time", or lie on the floor with him face to face. Another way to develop those muscles is to wear him in a sling while you work. Your constant movement forces him to constantly adjust his position to keep upright. Good luck!