There's probably a formula under which all the variables are taken into consideration. I think you need to go through the application process to determine what the VA will pay.
One's net worth is not taken into consideration at all when applying for service connected disability. He is probably receiving around 600/mth for this rating. When was the last time he was re-rated? If it has been 2+ years, it probably is worthy to ask for a re-rating exam and see if this will push his rating higher. If the results of that exam leave him at 40%, I would consider applying for and switching benefits to non-service connected pension plus aid and attendance which could bring in over 1800/mth BUT one's assets will come into play with this particular benefit.