From experience with my boys who were very chunky like Sandy - they were at their fattest at around 6-8 months, then stretched lengthways as they became more mobile. Their tummy and legs didn't really get much bigger. So if Sandy is like that then the M/L will last him a long time. I know it doesn't seem possible for something to fit at 2 years when it's snug at 7 months, but honestly it is! Freya is on the highest rise on her BGs, and they also fit Brodie (just on a looser waist setting).