I realize you have a question about social security. My goal is to provide you with excellent service, and help you better understand your options.
How are you this evening? I'm glad to help you with some extra questions that you have.
You are not expected to spend all the money you received for your son by a certain date. You should put this money in a separate account for the benefit of your son.
You can spend the money on anything that your son needs - food, clothing, school supplies, anything at all so long as it's for your child's benefit.
With the money that is left over you simply save it for your son. Each year you have to fill out an accounting form telling social security how you spent the money, and if there is funds left then you tell them that on that form. They keep track of this, and before he turns 18 they will ask you for any of the funds that you did not spend, back so that they can give it to your son when he turns 18. If you have an account set up in your sons name, you would show them that and they would let you give it to your son directly.
You could not take some of the benefits as payment for past expenses. You would have to improve the life of your son or care for his current needs. A good article on this is found, HERE
You can spend it on anything to do with your son. You can buy furniture for his bedroom. You can buy him clothing...anything he needs. Take him out and have some fun with it - get him a nice computer or video game console. Get him some new clothing. If he needs dental work, get that done.
There is a 9 month time frame in which some people have to spend back payments, but this is only for low-income benefits. If this is not a low income benefit then you do not have a specific time frame in which to spend it.
Please let me know if you need any more direction on this. If not, please rate my response as "excellent" so that I may receive credit for assisting you today. I do not get credit or get paid unless you rate my answer.