Your local mobility company may offer rentals. You could call to check that if he's not staying long and you have no need for a permanent structure. We needed permits here in Illinois to build wood ramps.

We have two sets of portable ramps, one 7 foot, one 9 foot that we bought. I found the 7 foot ramp on Ebay for about $240 and they work well. We have to help Jim up sometimes due to the pitch but overall they do the job when we have to go somewhere stairs are. The 9 foot ramps came from a mobility store but they were far more expensive, I think about $800 but they are much more sturdy.

And yes you are right. You must have a 1 foot pitch for every inch your structure is, in your case the porch is 24 inches. Mine was 18", so my ramp is 18'.