Liability waivers are helpful, but are not absolute. You can have your acquiantence assume the risk of liabilty from the normal hazards of brush clearing: for example, chopping into his own leg; heat stroke; tripping and falling; etc. However, you typically cannot have him waive your liability for conditions where you have been negligent or have intentionally done something wrong. So, for example, if you know there are dead trees, with a greater-than-normal risk of falling on him, but do not warn him, you could be liable for injuries from those trees. Other possible sources of liabilty for you are any animals you have on your property; any pipes or electrical wires strung through that area in a careless fashion; if the tools you give him to use are not well-maintained; etc.