Unfortunately closer to age 4 is often the time when kids are trained these days and especially for boys. Doubt you will get the parents on board even with the extra charge - if they pay it great but most likely they will pull him from care - be sure you can afford to lose the income.
At the same time your other option is to release the child using the excuse of unwilling to potty train and you do not feel comfortable changing diapers on a child old enough to do it themselves and replacing the child iwth an infant.
Unless the child is really big for his age haven't really found an issue as in much difference between a 2 year old and 3 year old except I love it when their head practically hangs off the change table and they have to support themselves. I change diapers on my 19 year old special needs daughter - now that qualifies as gross. Maybe that desensitizes me to 3 year old poop too much to be objective for you.