Can my husband enter Thailand on a tourist visa while waiting for his Non-Immigrant O visa approval?

Now to answer your question, CAN he fly back here, enter free stamp "visa exempt" and wait for the visa to be approved.

He could,

BUT

if the consulate sends a request for extra documents asking for copies of all his passport pages (blank ones too) <- which is something they can do and if they see he already LEFT Italy and is in thailand they will reject his application and he will lose the fee

I will leave it to you to decide what he should do,

He sure didn't have to go all the way back to italy to get that visa. He could have gone to a thai consulate in a nearby country and gotten the Non-O thai wife visa in just a few days

doubtful he was on a "tourist visa" or will be entering on one 😮

It's far more likely that he enters "free stamp" (visa exempt) entry for 60 days <- which isn't a tourist visa, isn't a visa at all, just a free entry based on his nationality

It's posted on the Thai embassy in Rome website that processing is 14 business days

(that's actually about 21 calendar days IF there are no holidays)