Baby’s tummy hurting

So my almost 3 week old was drinking only breast milk since he’s been born. But sadly I’m not making anymore milk (not enough to feed him). So last night we started feeding him just formula ( gerber good start gentle). But it seems to be upsetting his tummy bc he’s been crying all day and won’t let me put him down. And its not like him. You can tell its his tummy hurting and he grunts and strains like he’s trying to poop. Which he has done today a couple times but its like he’s been doing that all day. He just cries like he’s hurting. And I have tried gas drops, it seems to help for only a short period of time. So what are some good formulas out there to help this? Sorry I just don’t know much about the diff types. Haven’t really looked into it bc I planned on breast feeding for awhile but looks like I can’t do that