I saw a blog about exes ignoring on another divorce forum- you can also post in their forum too. The judge might get annoyed with him for messing around I guess. If he signed then he should be going along with that surely? Have you spoken to the mediation company you used? What did they suggest? I would think that yes, he would incur costs! and so he should.
Last edit: 15 Jul 21 by rubytuesday. Reason: Moderator's note - details of alternative forum removed, see T&Cs 6.1(c)
Agreement reached at mediation is not the same as a legally binding order. It would be relevant though - unless the financial circumstances have changed dramatically or the agreement was manifestly unfair.
If you apply to the courts you could certainly point to it and suggest it should carry weight.
Costs are very rarely awarded in financial cases but there is no harm in asking for them. I would write a letter before court warning that you will be applying for costs.
If you are paying a solicitor it isn't worth wasting too much time and money on this. I would instruct your solicitor to take a robust approach. NB you will need mediation sign off if you intend to make a Form A application to the courts.