S.E.
If he is growing just fine, you can simply let him stay with the size 1. It will take him longer to eat, but he won't fight.
Or quit the size 1 cold turkey. He will be upset for a while but he will get over it.
Or you can try slightly increasing the size of the 1's with a needle until they are the same as size 2's. But you have to be careful and examine the nipples as increasing the size may cause a tear in the nipple.