Keep it FUN for a 4-5 year old during the Dad time, just allow him to get comfortable on the skis and with the surroundings. Allow the ski school to have some time (no more than 25%) to do the teaching. Also alot of areas have a program where "babysitting" is offered with several small children playing together on skis. This can add variety to keep his interest going by being with his peers, and can be considered part of his ski school time.
Depending on his maturity and interest, if a junior racing program is available that can be used to add technique to his skiing. If the interest is there he will try and emulate the more advanced 8-10 year olds. And from my experience, the coach's with these programs really help kids with encouragement as well as technique.
Remember, to much teaching to reach perfection at that age will take the fun and likewise the interest out of skiing.
