Hi Gillian and welcome to the Forum.
You should not be affected at all by his IVA as there are no financial ties. However, once you move in together his IP will want to redo the income and expenditure account to ensure you are each paying your fair share of bills and costs. This could mean his IVA contributions could rise if he is currently living alone or they may not change at all.
Assuming that you move in with him, his expenditure would almost certainly change. Cooking and shopping together is always a bit cheaper than eating separately.
If his IP decided that he could now afford more into the IVA because of this, he might be required to pay a little extra depending on the exact wording in his proposal. But this should always be affordable for him.
Just bear in mind that you may be asked to supply details of your income and expenditure just to show that you're not a millionaire or something - you won't be asked for any money, but don't panic if his IP asks.
He is likely to move in with me as he is currently renting. How would my credit rating/score be affected if I wanted to say get a new car or get a loan? My costs are likely to increase next year as I have two teenagers left at home and the first will be going to University next year.
Since I started my IVA, my wife has financed a new car, financed a sofa, had her first-direct CC limit doubled, sucessfully applied for a 95% mortgage (hope to complete anyday now). We also moved house into rented accomdation via an estate agent. She does earn enough for all of this to be in just her name.
My IVA has finished now, but all in all the effects were rather limited.