Your attorney is the person you need to ask. Since your attorney knows the details of your son's injury & the potential for any future medical care & disability, as an attorney, I would not try to second guess his/her decision.
This is not intended to be legal advise or as legal representation. I am a California personal injury attorney . Be aware that every state has its own statute of limitations; and statutes & case laws that govern the handling of these matters.
These are questions you need to ask of the lawyer that knows all the facts of your case. Keep in mind that if you really think you need to wait to see what happens with your son's back, you can withdraw your son's case and file it when he is older. Talk to your lawyer about this possibility.
I'm sorry that your son appears to be experiencing some back pain.
Insurance settlements are the subject of negotiations, you can ask for more if you want more but it may be that you won't get an additional offer, but sometimes if you push for more, you'll get it - you should discuss the case with your attorney as he or she is in the best position to know about your case.
If your son's claim is not settled, it won't be closed until he's an adult (18) + 2 years (in most states) so the adjuster may be motivated to offer a bit more just to close the claim.
This response does not constitute legal advice and does not create an attorney-client relationship. Due to the complexity and ever-changing nature of the law and to the limitations of this forum, this information may not be adequate for your specific situation.
Your post lacks clarity and completeness. I don't know why you clouded your son's case with your own vague case. I wonder if your son was properly in a proper car seat and who appears to be at fault in causing the collision and whether it was a low-impact one.