What Buenchico and dzug are saying is essentially the same thing. To be completely pedantic, he cannot contest the will. However the husband (whom I assume to be the widower) will be entitled to make a claim under the Inheritance (Provision for Family and Dependants) Act 1975. As well as the s3 criteria pointed out by Chris, the age of the applicant, the duration of the marriage (and anything over 5 years is accepted by the courts as extremely persuasive), and the contribution he made to the home generally are factors taken into account by the courts. Another test is the deemed divorce test - ie what he would have received had the marriage ended on divorce rather than death.
Authority suggests (cases such as White v White etc which although are strictly divorce cases) that the starting point is equality of division of assets, but that is only a starting point because a spouse is entitled to "reasonable financial provision whether or not required for his or her maintenance" - which is the higher standard, so further factors are the lifestyle the deceased and the claimant had together. The deemed divorce test is difficult to apply because in divorce the courts have to provide for both parties but in death, they only have to provide for one.
It is absolutely impossible to comment on the likelihood of his success without seeing full financial information and the witness statements. Unfortunately, the specific instructions left by your aunt are generally seen as self serving statements by the courts and as a way of justifying leaving less than is reasonable provision. Unless there is an "in terrorem" clause (which attempts to prevent a legatee under a will claiming on an estate) these are just another factor taken into account. In any event, in terrorem clauses are fraught with difficulty. The executors and beneficiaries should take immediate specialist advice.