NEED Advice: Jammers swimwear fitting

Hi. I am a parent of 9 y.o. boy, with great swimming potential. :) For the past 3 years I've been buying jammers just based on the fit (and kept buying a size larger every year). Now, I've decided to actually check recommended sizing. He is very tall - 4'10". His waist is about 26 and every site states that for this waist size should be 26. I am so confused as my son is wearing size 30 and I am thinking to already buy size 32 (next season). I am looking for recommendation from coaches and professional swimmers! Maybe I am wrong with sizing and that potentially can be not so good for his timing. I am also planning to buy an expensive suit for his swim meets (Blueseventy NERO TX Men's Jammer) and I don't want to make a mistake in sizing. Please share your opinion! Thanks!! :)