Difficult.
I know someone whose daughter married without telling her parents, and although they made light of it at the time they admitted much later that they were hurt, not even to be told until several days later.
Equally, another friend's daughter wanted to be married but no wedding so she and her partner planned to marry at a register office, no guests, then attend a large rock concert. They both told their parents before they left, whereupon both mothers went spare and insisted on a 'proper family wedding', took complete control and spoilt it utterly for this shy couple, who really wanted no fuss.
It is your son's wedding as well as his partner, and you are not wrong to expect to be included. I agree that it is likely that the mother will turn up, with or without ex and second husband and that is not fair.
Last of all, what does the bride think?