I haven't been able to lately, but I've run with my kids in the past. My 8 yr old son could easily do about 1.5-2 miles at a slow pace, talking the whole time, hardly seemed like he was trying without any training at all except playing (he runs around outside a lot when he plays). Anything past 2 and he would start complaining.
So, I guess if I wanted to train my son for a 5k, I would run him a few times per week with me where he is already comfortable (1.5 miles for us makes a convenient loop), and after a week or two of that, increase one of the run lengths a bit at a time.
Both my kids struggle with maintaining a steady pace. Kids want to sprint then stop and hang out. That's not necessarily a big deal, but it is something I noticed was hard to do for mine.
That's great that your son wants to run with you!!!
I can get my son to run with me but my daughter acts like even the suggestion is pure punishment.