My 2 month old son arches back a lot and gets stiff?

so... he has a mild case of colic, and he is a very gassy baby. we burp him after feedings, and he is not constipated. he gets both breastmilk and enfamil gentle ease formula. whenever we lay him down, and sometimes while holding him, he will turn and stare to one side, and its difficult to get his attention for 30 seconds to a minute, like he is spacing out. also while laying down, he will roll to his side and arch his back and neck, and be very stiff. he cries a lot too. any ideas? i read up on this behavior and, just like the internet does, it gave me many causes. mental retardation, acid reflux, cerebral palsy, hialtal hernia, and the list goes on. i dont think its a delay because he is smiling, cooing, and lifting his head like he should, but i dont know for sure. i brought up hernia to his dr. and she said, no way to know without and xray, no treatment, better to not do the xray and see what happens. any input or suggestions appreciated. thanks