The results are... improvement in the first year 23 minute to 19:30 5K but consistently 19:30 since then (2 years).
For this year, I put him on a simple 5K plan leading up to a race at the end of April. Basically a fartlek or two every week with some hill reps and a 90+ minute long run every weekend. It'll be fun to see if that sort of training affects him differently or if 19:30 is the best he can do however he trains. Obviously, it'll be a one person experiment but still a fun comparison.