I try to avoid MDF for fret and scroll sawing to avoid wearing a dust mask all the time while working.
It's worth visiting the US-based Ocooch Hardwoods website.
I contacted them after finding out that that they sell up to 8" wide and up to 1/2" thick hardwoods in a great variety.
Admittedly...
Hi, a rather late answer but I'm struggling with the same problem these days.
If your son is really after sheets than the best option would be veneered plywood sheets.
Here in the UK I use mainly Hobbies in Norfolk to get mahogany, walnut and oak panels, 10 cm wide and 6 mm thick and usually...