Personally I would go and if it is picked up just say you did not notice they had forgotten to sign it. They should then check and let you through. That is my opinion. It is a lot of hassle and cost to reschedule your flights.
I agree he already has a multi entry visa, so he can basically leave and re-enter without issues. The bit I don't understand is you say he will get a new stamp with a new date each time for 1 years time. That does not make sense to me. If he gets a new 1 year later stamp each time he could keep the O-A going forever. It is only valid for 1 year, as per the original stamp of 8 Apr 2025, and he can extend that once to get 2 years worth. Is this not correct?
it seems strange that I can show money in an Australian bank / superannuation account to get my O-A visa, but that is not accepted for the extension. Instead I have to revert to having 800k baht in my Thai bank account for 2 months.
thanks that is what I thought, and was just looking for confirmation. I did wonder how they would treat the existing O-A visa situation. I think they will go for the money, cancel my O-A and issue O-X, but I will need to check.