In general flatter runs don't do well at that time of year because the snow is very sticky. It is essential IMHO that you take your son somewhere that does extensive grooming and salting. That means I would consider only Mammoth and Bachelor, and for this season Mammoth is definitely preferred.
What will be open for sure, groomed and salted:
Chair 2, 1000 vertical:
Stump Alley, wide average blue, fairly busy
Mambo, easy blue
Chair 1, 1000 vertical:
Forest Trail, easy blue, also with easier terrain park features
Unbound Terrain Park, the big hits and halfpipes
Broadway, wide average blue, fairly busy
straight off 1, past chair 23 to St. Anton, average blue upper, easier blue lower
Powder Bowl, Fascination, groomed single black, often used for racing early morning, thus excellent corn midday
Chair 3, 900 vertical:
Frontside will have at least one groomed single black
Backside Saddle Bowl is average blue with nice views of the expert runs coming down from the top.
From the back of 3 you can also ski the full ~1,500 vertical of St. Anton
Chair 11, 200 vertical:
2 beginner runs, In May they will still get sticky by late morning
With the deep coverage other sectors could be open, but the criteria will probably be how many people are there. Chairs 4, 10 and 12 give you a whole bunch more blue runs. 4 and 10 were open Memorial weekend 2 years ago and 12 was open Memorial weekend 1998. Midweek I would not count on anything more than what I listed above.