In this case, the Beneficiary will be able to travel and reenter the U.S. in F status as long as he has a valid F visa and the H-1B petition (with an October 1st start date) is approved. The Change of Status (from F to H) petition would be considered abandoned if the Beneficiary travels while the petition is pending approval.
It is also recommended that the Beneficiary carry proof of employment (job letter, pay stubs, business card of supervisor, job description profile, H-1B approval package, etc.) with the hiring entity to document immediate employment when he returns after his trip abroad.
We recommend premium processing in this case.