how long to give formula before switching?

KT
My LO is 3 weeks. He's breastfed but we want to go strictly formula. He's been on Enfamil gentle ease since Tuesday taking a few bottles a day. He's not sleeping at all, spitting up often, very fussy, not pooping as often, acts like he's in pain. So should I introduce simliic and if so what kind? 

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