Tawonga Huts is a long drive from Melbourne, somewhat exposed, a short distance, not much climbing, and a hut where you can hide from bad weather. Jaithmathang can be climbed, and if you are comfortable with a short off track foray, you can keep the altitude on the way in to the saddle west of the huts, then going down to the huts.
Erica car park is closer to Melbourne, the walking is sheltered all the way, StPhillack is a longer distance (although camp can be made earlier), there's a lot more climbing (it should be okay) and there's no hut.
On balance I like Erica car park to the StPhillack region. The distance and climbing can be managed by taking breaks, and the rocks should be fun. Depending on what time you start, camp should be reached by noon-2 pm. Despite any gripes at the time, providing the challenge is not too much, later he will relish the achievement.
I'd avoid both areas if the forecast is bad. Wet cold weather will not be enjoyable for a first walk.