Yes. He can challenge this in two ways. Firstly by a contentious probate claim on the grounds that the will is invalid (either through failure to follow formalities, lack of knowledge and approval, lack of testamentary capacity, undue influence). This is normally started by lodging a caveat at the Probate Registry which prevents the grant being issued. It can also be started after probate has been granted though by way of an application to revoke the grant and declaration of will's validity.
The other thing he may do is claim against the estate under the Inheritance (Provision for Family and Dependants). As a child of the deceased he is automatically entitled to make a claim - he does not have to show dependency (although if he does it will assist as one of the factors that the court will take into account).
There may be other claims that will arise, but these are generally rare. I suspect it is one of the two I have described.