Is the tester from the school district and was the testing mandated? If not, I don't see why you need to hold him back. In our very high performing school district, there were so many kids who did not read or write in english because it was not their first language and they did not know the alphabet either. They all seemed to pick up reading, writing and numbers fairly quickly in K.
I personally don't like the idea of keeping a child with no known issues in a preschool beyond age 5. My child was bored to tears in the preschool and he also had a late March birthday. In his K class, he was one of the oldest kids (a couple of them had Jan birthdays) and he was able to handle all the work easily (except handwriting, which was not taught in his classroom).
If your husband has issues with K entry, let your child review some "getting ready for K" type workbooks and perhaps some Singapore Math EarlyBird/essentials, Miquon and some handwriting practice in the summer.
