What Leadfingers said. He has lots of experience.

If you are using one of the internal amps to power the monitors (ie if the monitors are NOT self powered - borrow a separate power amp to power them and switch all of the amps in the Phonics to the main ("Front - of - House") speakers.

If the bass amp is a decent size, don't mic or DI him. Low frequencies really eat up the power. Borrow a bigger bass amp if necessary.

If the electric guitar amp will play stereo, do that, and put one speaker each side of stage, then you'll hardly need him in the front-of-house either (but only if you can trust him to turn down as needed).

Without the bass in the PA, cut the 40 Hz. The human voice does not get nearly that low. Bottom note on a 6 string doesn't get that low either (A=440 - bottom A string on guitar=220, A below bottom E ie A string on a bass guitar=110). Use the rumble filter.

Usually set the main EQ in a gentle grin (but in the present case, leave the 40 hz well down).

You can get the vocals to bite a bit more by boosting (in your case) 630hz and 1.25kHz, but it will un-sweeten the sound, so be sparing.

Use foam protectors on all mics to keep wind noise out.

If it distorts, turn it down!

The holes in the dustbin liners reduce the waterproofing and cones in speakers HATE getting wet - if it is pissing down, do not cut holes, and put up with the very slight sound reduction and occasional flapping of the plastic.

If it is raining make absolutely sure that no wet can get in to any mains connections. Your life depends on it.