Lactose intolerant 6 month old?

My son has always been difficult to feed. He gets extremely uncomfortable, cries, pushes the bottle away and it is a struggle to get any formula in to him. And afterwards he was always so uncomfortable and bloated, passing a ton of gas and then straining with every poop even if they were soft. We took him to multiple doctors and even saw a feeding specialist and an ENT. He was eventually diagnosed with reflux and put on medication, the medication only worked for a short period of time and then we tried a new medication and the same thing happened. We took him in to his doctor yesterday because he was refusing to eat at all and she suggested we try a lactose free formula. We had tried nutramigen in the past and it made him more gassy so i was skeptical. We bought enfamil lactose free and right from the first bottle he has had no issues with it and was calm and actually ate! So my question is, for those mamas with lactose intolerant babies, was it an immediate difference the second you switched to lactose free formula or did it take a while? Im still skeptical a bit because it seemed to immediately work. It seems too good to be true!