The saplings he tries to make from the wished for trees will wither and die. The multi-fruit trees can be grown, though! They'll act like regular trees, taking at least a year or two to grow to a viable size before they'll bear fruit.

If he wanted to wish for a little help getting his saplings to work for the other plants, it would be the same amount of points that it cost for the original tree minus 100 points. It would also take at least one year to grow to a viable size.