Beth and
to the forum.
If you are buying for your 6 year old then it need to be something he can reach now but still be enjoyable as he gets older. Plus it needs to be something that he can move around himself. Maybe not now but certainly by the time he is 8 or 9.
For an older and larger person most advice would be for an 8" dobsonian or bigger
However IMO they would be too big to move around on his own as well as carry out the necessary mirror adjustments that have to be carried out every now and then. The other option is a small refractor on an AzAlt mount. He will soon want bigger and better but he will never out grow it.
The best suggestion I can give is to go along to an observing night with one of your local Amateur Astronomy clubs and look through as many scopes as possible.
BTW what do you consider to be a reasonable price?