how to teach 4yo to wipe bum......getting ready for school

[miomaolalalalala]

not really sure how to do this, have asked dh to help ds cause i'm not sure if theres a difference in wiping between boys and girls because of anatomy (eeek tmi!).....but dh and i have a different technique (please bear with me!)and hes not usually at home when ds does a poo.
tonight was first attempt, daddy was home and ds as doing his usual half hand stand thing wanting his bottom wiped. daddy suggested he do it himself and ds got all fizzy about getting his hands dirty...even though he has been toilet trained for 18 months. hes not a squemish type so im not sure what the problem was.he attempted anyway and i put big smiley face on calender and have promised big present if he continues to try. hes just had 4th birthday and i want him to be able to do this come september when he starts reception. i dont want his confidence knocked and dh still remembers sitting on the toilet at school shouting "can someone wipe my bottom pleeeaaasee?" 30 years later