Questions
Your questions:
Should you have filed?
If you think he is disabled and you and his other parent have low income and resources, getting SSI is a way that you can get money to get him the special treatment he may need.
Do I think he will be approved?
You never know unless you file a claim. At age 6, he functions at the level of 3 1/2, according to you. That is a developmental delay. Also has a speech delay. Can't tell you here if it the delay is severe enough.
You can't afford an attorney:
Attorneys are only paid if the claim is approved. Generally, your contract is for 25% of retroactive benefits.
If he is denied on the initial application, there is an appeal procedure. It is important to be thorough on all forms. It is important to respond timely to any requests from SSA. It is also helpful for the reader if you put periods at the ends of sentences.
You do need to remember that if your son does meet the medical criteria, for the next 12 years (until he is 18) it is the parents' income that determines how much money he qualifies for. SSI is a public assistance program so there are plenty of rules about income and asset limits and reporting responsibilities. The more money you make, the less SSI he would qualify for.
It might be helpful to you to find a support group for parents of children with developmental delays. Your family can be supportive, but they don't feel what you feel about your baby.