My 18 mo old son is in the 100+ percentile on height and weight. I have to buy at least 3T or 4T to get the waist big enough. But he needs more room in the seat as he's still in diapers and of course the legs are way too long. I'm trying to sew pants for him but the patterns are sized too small for this little brick of a boy! Does anyone have suggestions on which patterns run true to the pattern envelope sizing and which ones might run a little large? Any hints for altering a pants pattern to add fullness in the straddle (especially the back)?