It is very hard in general to transfer into the COE from any major.
If your son really wants to pursue engineering, I would suggest he goes to UT. Your son would probably have an easier time transferring from UT-engineering to TAMU-engineering rather than from MAYS. If worse goes to worst and your son was unable to transfer from UT then he would still graduate from a great school with his desired degree.
Surprised your son didn’t get into the COE, however there were a bunch of great applicants with even greater stats that were deferred to Engineering at Blinn.