trustee and beneficiary can be the same person.

Successor trustee will become trustee if both parents passed away.  So what you are asking is:  can a trustee be the same as beneficiary?  

The answer is yes.  Trustee can be the same as beneficiary.  

However, the downside would be since your oldest son controls the trust for himself for a few years, in case during those few years of sole trusteeship, he's sued for debt (causing injury for others, credit card debt etc), the creditor can reach into the trust because he's both trustee and beneficiary (for at least 1/2 of the trust).