Diaper sizes

Hey guys, so my son is about seven weeks old, and if I had to guess, maybe 12 pounds? He’s a really chunky baby, he doesn’t have his next appointment till February 24, but anyways, he recently just moved up to 0 to 3 month, clothing, and he is starting to out grow newborn diapers. the diapers leave a ring around his belly, thighs, etc. and also doesn’t really fasten in the lines on the diaper anymore. but I had some one size diapers and I tried putting him in those, but they still seem kind of big on him. I have to fasten the straps a little tight so he doesn’t pee out of his diaper, so should I just put him in the ones? Or should he stay in the newborn diapers for a little while longer?