Without prying to much into your families private life I must say one consideration is how long your son will be living with you and where he will be going when he leaves. If he gets his own place in the coming years it may very well be hard, if not impossible, for him to take the bird with him - would you be prepared to completely take over care of the bird? If you cannot confidently answer yes to this then perhaps your son should wait until he gets settled into a home of his own to take on such a long term commitment of owning a parrot.
But, to your more direct questions: all parrots can be nippy. If you get a parrot you WILL get bit, they WILL draw blood, and it WILL hurt. However in a vast majority of cases such bites can be kept to a minimum, and with a Grey there wont be any loss of fingers provided you don't make an effort to stick your finger in his beak. One could not make such a guarantee with larger macaws which can be very sweet and gentle, but if they do decide to bite then can do serious damage.
With some patience and some work you will have a cuddly buddy of a bird, it isn't that hard - however occasionally some people are not able to be patient and not willing to put in some work, in those cases the bird can seem pretty unpleasant. In other words a parrot is a serious commitment but almost without fail you get out of the relationship what you put in.
I'll leave the 'where to buy' question to others as I'm not particularly equipped to give a good answer other than to get to know the source: check them out thoroughly, visit several times if not more before buying, and (as cynical as this sounds) don't be quick to take their word on anything. There are many wonderful people out there who may have birds for sale or adoption, but there are at least as many cruel people in it for the money.
Parrots are quite significantly more expensive than nearly any other pet, though most of the costs are 'one time' purchase costs. Maintenance costs are quite reasonable, some good food and the occasional vet check up is very affordable.
The 'price tag' on the bird itself will vary greatly depending primarily on species. I've never shopped for Greys but I'd suspect them to be in the ballpark of $1000 or more at "stores" while often being much cheaper from breeders, but still in the several hundred range. I have also heard second or third hand that Timnehs are generally cheaper than Congos - we do have several active grey owners though that will be able to answer this far better than I have.
In addition to the bird also consider the cage price (and perches, toys) - which quite often is more expensive than the bird. A good cage for a grey can easily sell for over $1000 at a major store, but again you can find much better deals from smaller operations.
All in all price can vary WIDELY depending on your sources - if you are patient and really do some leg work to shop around, are willing to adopt an older parrot (perhaps with some 'baggage') you could probably get all you need for a few hundred. If you go to the big-box pet stores it would be easy to rack up a bill of several thousand just to get all the basics.
So do your research, and shop around, but at the same time this is a lifelong commitment - spending a bit more now to get started on the right foot can be a very wise investment.