Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your fellow's surgery site is leaking watery, blood tinged fluid and thus your pup is licking his incision.
It is not uncommon for a seroma (collection of fluid due to bleeding and disruption of tissue) after an FHO. Between removing a portion of bone (which bleeds a lot when cut) and needing to go through and transect or tease through several groups of muscles (which also bleed a lot) to get to the surgery site it is not uncommon to see a fair amount of swelling and a post-operative seroma form.
But I share your concern about the incision becoming contaminated and infected if your fellow is licking. We need to stop him from licking right away. That likely means that we need to place an elizabethan collar (lampshade collar or "cone of shame") so he cannot reach his incision to lick it.
I would also recommend warm compressing the incision and trying to get the fluid to drain out. The less fluid accumulating the better as large amounts of fluid disrupt the healing process and can lead to breakdown of the incision and surgical site.
Apply a damp, very warm, clean washcloth to the incision for 5-10 minutes at a time 3-4 times a day until the swelling resolves and we aren't seeing any further fluid drain out.
Do not cover the incision. That will impair drainage and allow fluid to sit on the skin which further predisposes to infection and tissue trauma.
Best of luck with your pup, please let me know if you have any further questions.