potty chairs and training

My 4 year old grandson has cerebral palsy where he is very alert but cannot walk, talk or communicate very well.  Lately, he has been ummming a lot when he needs to poop.  He is currently in size 5 diapers (which are getting too small).  We are thinking of trying to potty train him but all the current potty chairs are very small and the deflector shields are even smaller.  We don't think sitting him on the toilet will work since, although he can sit up we don't feel he can hold on good enough to sit by himself.  I have checked some of the "special needs" sites and the cost of special toilet seats are almost $ 500.00.  If we could just find someone who manufactures a real old fashioned potty chair with a large deflector shield, we think that may work.  Any suggestions?

[ForTank]

My 5 year old has cp and is also potty training.   He is in school where they are very consistant at taking him to "potty" at the same times everyday.  Within 9 weeks he learned that schedule, but only at school.  He was very fortunate to recieve a potty system that fits over our regular toilet, however, my son who is also in size 5 diapers (that leak often because he pees like a big boy, heavily) is too small to sit on the toilet and be strapped correctly.   Therefore, making it difficult to be consistant with the potty training at home. I had the same thought about the old fashioned chairs but haven't had any luck finding one, Please let me know if you find a manufactor!  Thanks