We went last year at the end of April - it was the end of peak season, nowhere near Easter, and just before the start of the Celebration. Overall, the crowds were fairly mild, but that was last year.
We found Epcot to be the best park for that time of year - the flowers were amazing, as were some of the other parts of F&G festival. Even if you aren't into gardening at all - they make flowering topiaries of the characters that are just amazing to see! If I had to visit one park at that time of year, and I'd been to WDW before, I'd go there. Plus, there's a lot to see that doesn't require waiting in a line, and lots of great QS places to eat. But generally, I wouldn't try to do WDW in less than four days. IF I was in your son's position, I'd choose an extra day at WDW over a day at Universal...but that's me.
It also depends what day of the week it is regarding crowd levels. Certain days are worse than others - EMH days can be the worst. They should consider that when planning their tour days.
As far as a MK tour plan - I'd say stay as late as they can - the crowds thin out at night. I'd also avoid eating at peak times and eat on the run. I'd also stay on rides during the parade, and maybe even during the fireworks. (Splash/BTMRR are really cool during the fireworks!) After the fireworks, a throng of people flooded Tomorrowland during the EMH night.
Big Thunder is great ride to hit during the fireworks. Splash will have a short line in the am, and evening. Avoid Small World in the afternoon, PP will also be crowded then. (Though PP always seems to be crowded.)
If the park is open late, and they can pull it off, it might be worthwhile to take a nap in the afternoon, so they can stay later.
If they are getting to the parks in the afternoon, it might be worthwhile to drive if they have access to a car. Another suggestion (say, if they are at a moderate resort with more than one stop,) would be to catch the bus at the main bus stop near the lobby rather than bus stops on the back side of the resort. Esp, if it's a MK EMH night.
The ride board (with wait times) by the castle used to be helpful, but recently, they were VERY unreliable.
I imagine your son won't want to hit rides like the Tiki birds per say - as opposed to the mountains, but if they want quantity some rides to consider would be: Tiki birds, HM, CBJ, Stitch, maybe Buzz, Snow White, AC, teacups - especially in the afternoon, when crowds are at their peak.
Otherwise, I'd say they have a good shot at getting at least a few FP's -I'd probably pick SM for a FP as soon as they enter the park. Late at night, they may be able to ride the other mountains multiple times without long waits, but not SM.