Can I collect back child support if my ex-husband did well and had restricted visiting rights, now that our son is 26 years old?

Why haven't you pursued it before this.

If he hasn't been paying back support all these years I don't think you can just one day decide to go after back support if you haven't been keeping up on it.

Can I collect back child support if my ex-husband did well and had restricted visiting rights, now that our son is 26 years old?

No. Your son is beyond child support age. All you have are whatever orders there were in the past and the entitlement to any support due under those orders. If you never had any orders it's too late. If you had them, it's too late to modify them.

That's about it.