First I applaude you for the four kids under the age six...wow! My advice would be the classic saying if you can't beat 'em, join 'em. Behavior like this is only annoying if it creates negative attention. You timing it is a wonderful idea because it creates a space in which he can just let loose (no hold bar) for 10 minutes or so without threat of it being unwanted. I'd be careful with the allowance though meaning, I'd let him know what comes next and only allow him to be "obnoxious" for a set time, then have him do something else. For example, you can play like this for 10 minutes but then after that, it's homework, then you can relax for bit. I think as much routine and structure you can provide, even with the silliness than you should.
Also, take 5-minutes out to try getting a bit crazy with him. Jump around with him for a moment, letting him know your interested in even the silly things with him. He'll appreciate it and may stop sooner just because he knows he can get your attention outside of acting silly. When it's all said and done, with 3 younger kids around, I think he misses his mom more than anything.
A little about me:
Only have the one (3yrs old girl), yet I keep my sisters two kids who are (4 & 2) often and notice the struggle for attention. Hang in there, and know that things only get better. C.~