Hi B.. I do agree with your pediatrician. I think that even though he doesn't like the tummy time, it is vital to his development. I would try laying down on your stomach in front of him so he has to lift his head to see your smiling face. Put toys 'just' out of reach of him. Sing songs. You might even have to leave him on the floor, but in the same room so he knows you are still there, and let him work out his frustrations a few times.
Also, as far as arms strength...I always pushed my kids up on their hands (supporting them) and then let them go slowly so they could develop the 'idea'. Of course, they flopped back down, but not for long. I also would have them pull themselves, while on their backs.
It's not like he won't develop those skills eventually (we hope), but if he does have a delayed disability, this is when they are looking for milestones to see if something will need to be done in the future. You do want the best for your baby, so watching them struggle is just part of the learning process. It is hard to watch, but is necessary at times.
Oh, good luck and I hope things get better for you quickly.