?SPACERS

I am not sure if I am in the right area.. my 7 1/2 year old has had issues with his mouth.. he now, well for the past few years has had a wonderful dentist.. my son has had a few surgeries. one which led to an abscess on the roof of his mouth when he was about 2 and was told by his first dentist it was bby bottle rot (which he was on a high calorie formula diet for maintaing or weght gain due to his heart.. so we had to go to a peds dentist who discovered it was an abscess and he lost the 4 upper teeth... he has lost the 2 middle ones on the bottom normally and when they came in pushed another one out on one side and 2 out on the other side.. his two upper front teeth have come in and and now there is no space for the other 2 on the uppers to come in and no room on the bottom for the ones that were pushed out... his dentsit said he will need spacers...
my question is at what age do they usually put spacers on and for how long approx.. would he need them... I know his dentist is the best to ask, but I was wondering if I could get a heads up for the cost of them i need to know how soon  i was told by 8 maybe 9 years old
Michelle