Am I entitled to my security deposit back if I move out and someone else take's over the lease as head of household with the landlord's approval?

Not by law you aren't. We need some more details to refine the answer. We need to know about any early "lease breaking" provisions in the lease. Normally you are responsible for the remainder of the lease until a new tenant moves in. That does not mean you automatically get your deposit back. The LL can deduct for damages to the unit, cost to advertise the unit, any loss of revenue caused by your lease breaking.

Well I removed myself from the lease and he approved my son as head of household. There were no damages when I moved out. He never even came to do a walk through.

In this situation it sounds like he allowed your son to take over your lease, particularly if a new lease was not drawn up and signed. In such a case he would not be obligated to return your security deposit. The deposit would be assigned to your son and your son would be responsible for reimbursing you for it.

I totally agree with Lisa. The LL may have allowed you to leave but unless your son steps up and makes his own deposit your money is still part of the lease.

Yes, there is a deposit, now under your sons lease. If your son had taken a new lease, if he had given his own deposit.

Since he now has control over your deposit, you could ask him to pay you that sum. But it does not sound like the landlord should give it, since the same lease is still in effect