Hi and welcome to HCM.
Of course this is not cancerous. It is commonly found after surgeries because fluid or blood accumulates inside testicles. This wont cause any serious problems and it should pull back in few weeks. If it doesnt happen it can be punctated and aspirated easily. He should just try not to stand a lot and watch for some warning signs such as redness
,pain and urinating difficulties. Wish you good health.