K.P.
It sounds like you are already doing quite a bit! Lucky kid!
Why do you need an "entertainer" at the park? With 8 boys, get the wiffle ball set out and play 7 innings... serve burgers and dogs along with cake and some Gatorade. Kids don't need to be "entertained"...do an outdoor Olympics with relay races, traditional party games and some pizzas.
He's only 5, so keep in mind that you are setting a precedent for all of his birthdays (and siblings). You may want to consider ratcheting it down next year. Do one party at a park and have a ton of games & activities planned.