I'm not a lawyer but do have some general info on this, having consulted my lovely, fragrant mother - a former probate legal exec.
Firstly, anyone can contest a will so there is nothing that can be done to stop your brother making a claim if he wishes. But that doesn't mean he has a chance of success.
The main reasons for contesting a will are:
1) That one or both of your parents lacked mental capacity at the time the will was made. Assuming your parents do not suffer from any illnesses which are relevant to this then it is unlikely to be an issue.
2) That one or both of your parents were under undue influence when they made the wills.
3) That the will does not meet statutory requirements. Unlikely to be a problem if drawn up by a solicitor - the will would be duly signed, dated and witnessed.
One way of addressing issue 2 is for your parents to have new wills drawn up including a clause acknowledging your sibling and explaining why he is not a beneficiary. They would have to pay for this but it should not cost very much as the bulk of the work has been done. Unless your brother can prove he is financially dependent on your parents - for instance if he is ill, handicapped or perhaps a recovering drug addict - then he stands little chance of making a successful challenge.
If he does contest, the costs of the legal proceedings would come out of the estate. Depending on how vindictive he feels, he may decide to contest the will just to diminish your inheritance.