Just enter him as a dependent as you normally would. You can check the box for disabled but since he is under the age of 19 it will not have any affect on your tax return.
Claim your child as a dependent on your tax return. But...there are no special credits or deductions on your federal tax return for saying that your seven year old child is disabled. Your state laws might be different.
If you have medical expenses or expenses for special schooling, etc. as a result of the issues, you can enter medical expenses if you are itemizing deductions.
You can claim your minor son as a dependent for purposes of the Child Tax Credit. Claiming a child as a dependent generally requires that they live with you for more than half the year, they don't provide more than half of their own financial support, and are under the age of 19, or under 24 if a full-time student. Please see this tax tips article for more information on claiming dependents.
If your child eventually receives Social Security payments, they are the child's income and not taxable to you. See this help article for more information.