Increasing Size

Hi All, I really like the attached pattern and want to continue using it as my nephew grows. He’s currently just turned 1 yr does anyone know how many stitches I’d need to increase by to make this pattern in a 12-18 mth or 1-2 ? Or do I just use the largest size and increase my needle size ?

It looks like a fairly standard crew neck sweater. You can try using a heavier weight yarn and larger needles but you’ll probably have to tinker with general proportions. Best thing to do is look at average size charts and work some swatches to figure out how many sts to cast on for body and sleeves with your yarn and needles.