Its all about the intent.
As he has a pending visa application (K/finance visa by any chance?) that shows an intent to stay in the US in the future...
As such , as immigration official may deny him entry.
If you have married an american in the states,as long as you entered the country legally
with no intent to remain at the time of entry you can adjust status from within the country and basically never leave , so they may deny entry because of that- they may see him as trying to bypass the visa process he has started and him attempting to fast track it by just staying there.
I have to wonder why on earth he applied for a visa before he was married....when its a hell of a lot easier and quicker once you are married, both from within the US and outside of it, as you are in a different (priority) visa category.
He could always cancel the application?
He's also rather worried about being denied entry and then never being allowed in the country again.
Dont worry about that...once you are married to a yank, it doesnt really matter. You could have been living there illegally for 50 years, or denied entry at some point, but the slate is wiped clean after marriage and you are entitled to apply for residency as you are married to a citzen.
Would suggest going to see an immigration attorney...