Joanne - The sizes are tough. My son was 18 or 19lbs at 6 months and was wearing mostly 12-18 months (or some 6-12 months if they ran big). That was in February. That summer he still wore mostly 12-18 months, but could do 18-24 months too. He was 10-12 months over that summer, and was about 31" and 21lbs. So his growth really slowed from about 7 months to a year.
Of course, he slowed down even more, and the next summer, when he was 22-24 months, he wore the same stuff from the previous summer, all 18-24 months. I didn't anticipate that, so I had bought all 2T stuff on sale the year before, and then he couldn't wear any of it. Worked out though, as the summer that he turned 3yo he was wearing the 2T stuff, so at least I didn't need to buy anything that year!
And I have to add, your son is way bigger than Scarlett and she's almost 13 months old! She's not quite 17lbs yet and still wears pretty much all 6-9 month stuff, but still fits into a lot of 3-6 month stuff too.