I find with these little ones and yours looks very alert...that when they are on their stomach they can't see all the action and are frustrated by that. I used to make a bolster for DS. I rolled up a towel and put hair elastics/scrunchies at each end to keep towel together and then propped his chest on the bolster so now he was up and looking around. It was better, but he still loved his back or sitting assisted better. Once he was sitting at 6 months then he became more interested in tummy time. Then when he began crawling at 9 months, he hated being on his back! Ahhh, babies, they are so full of surprises!
HTH,
Aisling x