Up until 2013 (where Parental Leave was 14 weeks); the default option was for the leave to be taken in one uninterrupted block. Where an employee only wanted to take for e.g. 8 weeks they could not at a later date request the remainder.
Now with the increase in duration (since 2013) the default option is for two blocks that must be a minimum of 6 weeks each. So in your Husbands situation he should be able to take two six week blocks; However after that he has no legal entitlement to be able to use the remainder.
So in summary your Husbands employer is not enforcing the legislation correctly, your Husband should be able to take 2 distinct six week blocks but he then has no entitlement to the remaining 6 weeks.
In my place of work people have been able to avail of a day a week every week or easter / xmas as parental leave. I know that in my other halfs' (large institution) they are now allowing only 2 distinct blocks.