do you have a sample of the roof color you can use to pick siding color? some gray roof shingles are more blue, some more brown or green toned. It would help to know which one you're getting.

As for paint, I'd do a really good primer. (can you power wash the siding first and then encapsulate it w/a good primer?) Perhaps researching which primer would be best for that type of siding would be helpful?

Make sure any of the wood trim is sanded smooth to remove any flaking or old paint, and a good exterior wood primer used for that.

As for colors, A blue gray is fine (if the roof is a bluish gray) w/a nice bright white trim. what about the front door? can you paint that too?

So if you did an actual gray/charcoal roof tile,

I'd go w/something cool looking like the Wrought iron on the siding, White dove trim, and a bold front door color. Or, reverse it. try the light siding and do a dark trim color.

If the gray roof is cooler toned, w/a blue undertone,

try these

You could do the midnight blue on the trim around the windows.

more of a silvery blue gray roof tile

you could do the white trim, foggy day siding, the red for the front door: (plant some red/white flowers in the beds)

you like this?

I like Behr paint. what about this color? you could do a Satin w/a crisp white. (if you get the bluer gray roof shingle)

Or, maybe he would like more of a navy color w/white trim?

Hale Navy an Pure White, w/a Canary yellow door

These are warmer colors and whites that would work w/a warmer roof tile

this is a grayish/brown roof color

warm charcoal aspahlt shingle.

the warmer white on the trim would look like this, with this color of blue on othe siding.

Some other color combos. Show these pics to your son and see what he likes

And, if you got the gray asphalt shingle w/the greenish undertones:

you might be better going w/these colors