Is there anyway to downsize the project? Or see if another group could tackle part of it (eg he does the garden and say 2 benches as his project, maybe the Lions Club or whatever could do the patio part? Or something like that.)
Does he have an Eagle mentor to ask what boys have done in the past? (Or some one on the troop committee?) There is, of course, BSA paperwork that needs to be filled out for approval before he does a fundraiser.
In-kind donations are definately a help. Have him approach smaller, independant garden/landscape stores. IFWIW, in my experience, Lowes/Home Depot will only give a flat 10% for scout endeavors. The smaller places are usually more willing to work with volunteer groups. (Have him think of a way to recognize them- a scout in our troop recently did a garden area and made a plaque of all sponsors. If the group home has some sort of newsletter or annual report, perhaps they could be included in an article there? Etc..)
The best fundraisers our troop has are bottledrives, we can easily clear $800 in a day. Disclaimer though that our state has a bottle deposit law, so a heftier profit than states without that.
DO NOT let him do any sort of raffle as a fundraiser. BSA regs specify that fundraisers must provide a tangible and fair product for money taken in. (I forget how it's worded.) In other words, you can not take money for the potential of a reward- eg gambling. We had a boy who raffled off some pies he had made. He got chewed out at his final board of review with council. I felt bad someone had not told him that wasn't ok.
Good luck to him!