1990 GMC Sierra Engine Running @ 135

I live in FL and my son which lives in IL is having some overheating issues. He recently purchased the truck (161K miles) from a one owner co worker. At the time of purchase, the engine temp was fluctuating as driving. The coolant was very dirty so he had the coolent flushed which made truck run hotter. He also changed thermostat. His mechanic advised a new temp sensor and gauge which did not solve problem. Myself being 1000 miles away contacted a different mechanic. My mechanic stated the coolant was very dirty and gummy. He did another flush and replaced radiator. The temp did drop however engine is still running between 130 and 140. I read in one of your articles, a corroded impeller good cause overheating. I am thinking because the radiator was extremely clogged and a ton of sediment at the bottom, the water pump housing and/or impeller could be affecting circulation as well. When mechanic changed radiator, I requested for him to change water pump while he had open however he thought I would be wasting my money. Any FeedBack would be appreciated. Thank You