I was a solo parent for many years and my best advice is to establish a good support system. I was lucky and had a large family that helped me a lot (still does, actually, even though DP and I live together). Teaming up with other moms who can support you and be someone to lean on is a huge help.
As for the CS, I'd get an order for payroll deductions, if you don't already have one. If you do and he keeps switching jobs, then there's not much you can do. Check with the agency you're applying for aid from and see what "CS" you have to claim, though. If the order is for $200/mo, for example, and you only receive a fraction of that on a semi-regular basis, then you might not have to claim it. Also, if the amounts he's paying are sporadic and for specific items, say you tell the ex "DS needs $20 for some new shoes" and he gives you only $20, then you could say that's not CS, but he bought your son shoes, etc.