Hi there,
Some points to think about for when you're both here and in the meantime for him to 'start' building credit here in addition to what's been mooted so far
1. Get him registered on the electoral roll - once you are here, you as well.
2. Get a 'pre paid' Credit card and start to use it - Paypal for instance do one.
3. Speak to the bank and see if they'll allow him (and you once your here) a credit card with a very very low limit - £250 lets say. The trick here is to buy something say £150 worth in the first few months and ENSURE it's paid off before the end of the month, if you can work an element of your budget in that figure to 100% make sure that happens, the credit agencies will like it!
4. If he can get a mobile phone contract say at a tariff of £10 or £15/month that'd be helpful in building a rating - ensuring he pays it every time and regularly.
5. Having a BT telephone line also helps - having a phone line attached to a physical location/address where it's 'likely' you or him can be expected to be reasonably contacted will look favourable to the credit agencies.
6. Don't apply for 'too' many store cards and credit cards and any loans etc - the amount and frequency of applications and possible rejections is noted - stay sensible here
7. Other elements can help, for instance if he puts the electric bill, or Internet service bill in his name at the address and pays in full regularly with no defaults they'll all help.
Other elements apply if you have existing accounts and debts etc - and from what you've said he has none of those at the moment in his name. When you do marry, it's wise from a financial point only to maintain separate accounts where possible, becoming 'financially' linked to another person can have adverse effects on your rating and vice versa - obviously this element is personal to you but be aware of it i.e if one person does have a poor rating (for whatever reasons!) then it'll affect you - do some reading up on this.
With the store cards thing - be very careful to pay off as much as you can if not the 'full' amount, the interest and APR charges they have are extremely high, but use them carefully and they can help your rating.
In a nutshell, you are trying to maintain a stable and good repuation in that you are careful with your money, debts and repayments.
hope the above helps!
Cheers, DtM! West London & Slough UK!