Sometimes you can get interference on divisions of frequency (sideband bleed) like around 1200MHz (0.5 * 2.4GHz) or around 600MHz (0.25 * 2.4Ghz). It depends on power and cleanliness of the amplifiers (if any) and also antenna types, both can cause spikes into sidebands. Does he just get white noise ("static") or is there a modulation to it (more like a buzz or pattern)? What frequencies is he running on in VHF, do they come close to a division of your frequencies? You can also get a cancellation interference from running dissimilar frequencies near each other, such as 2.4GHz and 3.3GHz physically near each other could generate a "beat" on 900MHz as their modulations sync periodically. Odd reflections can do much the same, or nearby sections of un-earthed metal can be excited by a signal and generate their own noise on a frequency based on their length (sort of like a passive repeater), so you could look for nearby bits of metal that are on his wavelength or at half or quarter of it and make sure they are earthed or otherwise shielded. Even shielding and grounding sometimes doesn't help without multiple earth spikes driven into the ground far enough apart, and if the soil is not very conductive you may benefit from soaking the area around the spike with salt water periodically to increase conductance. My microwave oven (which you would hope is well shielded as possible) would knock out my Internet link whenever I was heating something up, back when I shared a neighbors wifi from 150 meters away - of course I was already on marginal SNR so any noise brought it down, but still made me go "hmm".
I also used to live next to a highway and would pick up 27MHz CB transmissions on my computer speakers now and then, either due to the trucker running a dirty multiwatt amplifier, or the lengths of my unshielded audio wiring (some of which were probably 102" or quarter wave) were able to pick it up and amplify it even without an actual tuner/receiver circuit. It used to startle me with the sudden voices from nowhere. 
