How can an EB-5 dependent stay abroad for two years without reentry permit issues?

The USCIS allows for temporary long-term absences in order to fulfill educational and military service obligations. It is safest to apply for an I-131 reentry permit ahead of time as preemptive indication to show that USCIS acknowledges your son's commitment overseas. It is recommended to consult with an experienced immigration attorney both to file the I-131 application as well as formulate documentary evidence for your son to carry when he reenters the U.S. on his green card after his absence.

You would 100% need a reentry permit if your son is planning on being outside the U.S. for more than a year consecutively. If the reentry permit is expired you could face issues upon reentry to the
U.S.

He definitely needs to apply for a Reentry Permit while on U.S. soil.

In general, applying for the reentry permit before leaving would be best. If the person can visit the U.S. during the time of his military service, he could then apply for a new permit when the old one is close to expiring. He should file U.S. tax returns on his worldwide income, and try to maintain as many other types of connections to the U.S. as possible.

If he has conditional permanent residency, he absolutely needs to have a re-entry permit if he's going to be outside the U.S. for longer than a year (or even 6 months if you're conservative). Otherwise USCIS may presume that he intended to abandon his conditional permanent residency if he stays outside the U.S. for that long without a re-entry permit.