My son is a big boy, at 17 months is barley fitting into new 24months clothing...whats next?

You will probably find that he will fit best into 3T. 2T and 24 months are different in size. 24 months will fit bigger in the bottom area to accommodate a diaper whereas a 2T has less room in the legs, bottom, and waist. If you are in an area where it is becoming warmer, I suggest you try one piece outfits in a size 3T since outfits with a waistband, such as shorts and a t-shirt will likely be a little big around his waist right now.