I think 25 miles is totally doable. I would just plan a halfway stop where your child can have lunch and just more around a little. I put a standard kids bike basket on my kids tag a longs and tether a water bottle inside the basket. I make the tether long enough for the child to get a drink, but not so long that is would drop and get in the tire. I also often let mine bring a favorite stuffed animal, that I tie into the basket. I don't know why, but one of mine really needs that stuffed animal in the basket.
We also have bells on our tag a longs and we all ring them when we cross streets, go over bridges, see purple flowers, (sometimes orange flowers depending on the season) and when we see deer and horses. The kids seem to really watch for whatever is on the list that particular ride and it seems to make the time fly by for them.
Also be aware of things like sun and wind. I find that my little ones often need to have long sleeve rash guard shirts to stay comfy on longer rides. They offer 50 SPF and wick away moisture so the tend to be more comfortable than uncovered arms in very sunny conditions. We can't seem to get enough sunscreen on my fair skinned kids and don't always remember to stop every 1.5 hours to reapply sunscreen as we should, so the rash guards are great.
In addition for that length of a ride you might need to have him in spandex type shorts with no undies. I have found that undies seem to give my kids blisters when we ride past about 15 miles. So we have taught our kids to ditch them for bike rides. It is so funny, that now we say we are going for a bike ride and everyone strips down in anticipation of getting their riding clothing.