Tennants in common HELP

We are about to sign the solicitors paperwork to be tennants in common with a 50/50 split even though I'm putting in less. In reality it should be 65/35, however he said no that he wants it this way. Anyway today he said 'it doesn't matter because if we split up let's say in a year or 2 then he can dispute the 50/50 and show that I contributed a lot less than him. We are also going to be joint names on mortgage contributing 50/50 to the bills. I can cover these easy my half but he does earn a lot more than me.
Annoyingly we cannot get an appt with the will people to put other wishes in place but more importantly I don't want him to be able to dispute the tennants in common.
Help... Can this be done.