I'm the mom of 3 boys (5, 3 and 9 mos.). I'm a teacher of piano and french horn, and our oldest began piano in August! As a parent of a child learning piano, and also as a teacher, the age where your child can sit (relatively :) still for 30 min., give or take, and is excited, and eager to learn and practise, is the perfect age! I began myself when I was 4 years old, and like I already mentioned, my husband and I began our son just this past August. He practices every day without prompting, and it's often the first thing he does as soon as he wakes up! Now, both my husband and I are musicians, so the apple is not falling far from the tree, but it is important that you don't push him to practice, although some encouragement is a good thing! Also, make sure you find a 'good fit' in a teacher for your son. That is almost half the battle. Ask around, take him for a demonstration lesson, etc.
With regard to the keyboard, that's what we have right now. The 'touch' (how it feels to play an electronic piano VS. a piano) is different between those instruments, but not a detriment until your son becomes much more advanced and committed.
AND, you can get a very good piano for much less than $8K.
Hope this helps, and good luck!