FLR (M) visa question help

Hi all
We are filling out the FLR (m) visa
1 questions asks about non applying family members,
My wife has a son who is staying in Thailand, we give his details and his address and guardians, but then it asks when he will be arriving in the U.K. ?
We wrote down that he is staying in Thailand but it still asks for arrival date and we can’t move forward without giving a date.
Any advise appreciated
Ps this is the 2nd stage of the 5yr spouse visa route,
My wife has been in U.K. for 31 months
We chose family option when applying for the extension.
Many thanks
Richard