Recent research on gout (what little there is) has debunked the myth that foods high in purines (typically and incorrectly thought to be highest in red meat) are the actual cause of gout attacks. The actual problem has been closely linked to dietary alcohol (ethanol) and fructose, which can cause insulin resistance, which impedes kidney function, thus the body's inability to excrete sufficient amounts of uric acid. In addition to animal-based proteins, Purines can be found in significant levels in a lot of plant-based foods. Even vegans can and do get gout.
But if this was gout, I suspect the description would be a lot more explicit because while a gout attack in the foot does cause swelling and "soreness", the pain is usually excruciating, not just "sore". Whether the OP's BF's issues are caused by red meat though, I have no idea. The only way to find out is to cut the red meat, and only the red meat, out of the diet for a sufficient length of time to see if there is a change. If there is, add it back in and see if the issue can be repeated. It may not necessarily be "red meat" in general, but a particular type of meat or additives in the meat depending on what kind of meat it is and where it comes from.