DS straining, should i change formula?

it seems like my DS in the past few days is always straining to go to the bathroom... he does poop eventually so not sure if i'd consider it constipation, but it seems each time i feed him he stops eating to try and poop and strains a lot, sometimes he goes, sometimes not... i also noticed he's become a lot more fussy. i've had him on gentlease RTF for about a week now (hospital originally started us on enfamil newborn) since i noticed he was gassy, and he was doing fine at first so i'm not sure if this is formula related, but now i'm not sure if i should try similac sensitive RTF to see how that plays out? not sure if the two are pretty much the same. I did ask my ped the last time i took DS there and he said if i switch brands, i should stay with that brand going forward. My only options are RTF because the ped will not let me try out powder until 4 months.. just wanted to see if any moms have been through this. TIA!

DS straining, should i change formula?

Is he straining--just pushing--or crying out in pain as well?

I think constipation is the consistency of the poop, not necessarily the frequency/amount.

I'd stick with this new formula another week before I switch again, especially if he doesn't seem to be in great pain. His body could just be getting used to the new formula.