Using a big toilet

Hi all

For months my 3yo son would quite happily use a big toilet whilst I held him when we were out and about, but over the past couple of weeks, for no obvious reason,  he's been refusing to do this and gets almost hysterical when I try to get him to.  I'd prefer to not have to start carrying the travel potty or seat insert around again if I can help it...does anyone have any tips on how to get him to use the toilet again?  I've tried reassuring him, reminding him of all the tinmes we've used it before with no problems and even offering bribes but nothing seems to work!