DD had a flat head and we've had a good success at rounding it out without a helmet.
I would say whyt your ped said was good advce but maybe a bit unrealistic. Did they say anything about how long you would have to do all of this for? I ask cos a babies head dosn't fuse together till after 14mo, so there is alsways a risk that if you fix it now it could falten out again in the future. I'm not saying don't do what he suggested, you want to round it out know any prevention is definately better that sorting it out tin the future. But i don't think its worth dramartically changing your life as you mentioned.
for instance, not going in the car seat. I would still go on long journeys, but give intersing things to look at either side so he moves his head round whilst in the seat. When you get out the other end make sure he gets lots of tummy time and be careful how you hold him. but don't not go, thats not fair to baby, grandpa and also you!!!
wedges are a good idea, but he will need to be turned regualry and spend some time on his back or else he'll get flatter at the side of the head!!!
I would loko at waht he suggested and then adapt it to living your life in the real world, don't be scared to put him on his back or in his swing if thats what he needs/wants, or gives you ten minutes peace to have a shower/cook dinner/have a rest. It won't do him any harm.
Ask the dr what his long term advice would be, realistically. It will get loads better when ds can sit up and then wants to crawl and evenutally walk as he'll be spening less time on his back anyway. But i do sympathis with the tummy time thing, DD hated it and i couldn't force her to lie on her front, she's scream all the time!!!
Hugs to you, try not to worry too much, its will get better with time, but try not to stress yourself and him out with drs advice.
Kxx