Unfortunately, your husband wouldn't be able to qualify for UK citizenship unless either he or his father were born in the UK... because UK citizenship can only be passed down to one generation born abroad.
So, if his grandmother was born in the UK, she should be a UK citizen 'otherwise than by descent'.
Assuming your husband's father was NOT born in the UK, he qualifies for UK citizenship 'by descent'.
However, only people who are UK citizens 'otherwise than by descent' can pass down their citizenship to children born abroad, so while your husband's grandmother can pass his UK citizenship to her son, his father cannot pass his citizenship down to your husband.