Sherlock (who is in his first year at Uni?) starts dating John's (who is in his 40s or 50s) son Victor (or whatever you wish to name him) but is soon introduced to John and realises he has the hots for him. John is unhappily married (or a single dad) and he and Sherlock start secretly dating on the side and eventually fall in love.
Age difference, cw: infidelity
Sherlock (who is in his first year at Uni?) starts dating John's (who is in his 40s or 50s) son Victor (or whatever you wish to name him) but is soon introduced to John and realises he has the hots for him. John is unhappily married (or a single dad) and he and Sherlock start secretly dating on the side and eventually fall in love.