As I read it, as long as his total service with the company is over two years, your OH's right to redundancy pay (should the situation arise) is unaffected:
http://www.acas.org.uk/index.aspx?articleid=4587
However, as he's only on monthly contracts, the employer can terminate his employment at the end of any such monthly period (but still with a duty to make a redundancy payment if he's not being replaced).
For definitive advice, call the Acas Helpline on 0300 123 1100. It's available from 0800 to 1800, Mon to Fri. Calls to 03 numbers are charged at the same rate as those to 01 or 02 numbers, both from landlines and from mobiles. (In practice that means that the majority of people can call free of charge, using 'included minutes' on their mobiles or if they've got a suitable landline calling plan). People have posted on AB to say that it can take quite a long time to get through to that number but that it's worth waiting because the advice is excellent.