To be honest I don't think they will take his income into consideration at all. I could be wrong but I am also self-employed and I remember enquiring to 2 or 3 banks about 12-18 months ago and they weren't interested in talking to me, and that was just to switch my current mortgage.
I know there are some brokers/banks that will take it into consideration but you may end up paying a higher rate.
However, like I said, I'm not 100% on this, and it was a while back so things may have changed. I suppose the best answer I can give is gather all the info you can and prepare it very well and then just sit down with them and talk.
Best of luck!