I haven't seen any male answers here yet...and I see the kid-growing-feet thing has been pointed out.
My 1/2 cents worth - soccer cleats for flag football will be ok. Unless the other boys are wearing football cleats.
The cleats and support are part of the discussion of course - but don't underestimate the effect on your son if he's the only one wearing soccer cleats.
I grew up as that kid, and was constantly embarrassed that my soccer cleats were my football cleats, my baseball cleats, my softball cleats, my tennis cleats (just seeing if you're reading).
Used football cleats would be great, as nobody would ever know. But everyone will know he's wearing soccer cleats and not football.