Some basic spots to start with might be areas where there is sand. Most sand lots are the easiest to dig in and a good place to get a feel of detecting. Try and get a sand scoop. You can find, inexpensive ones, on eBay. Basically, they are a scoop with large holes so the sand will fall through but the target item should stay in the scoop. Scoop up some sand and shake it out. I am fortunate that I am up along Lake Michigan and there are a lot of beaches here. Also, because of our proximity to sand, many of the Tot Lots, Volleyball Courts etc. are covered with several inches of sand. Gives me a lot of areas where I can search without hard digging.
Some other possibilities might be Play lots or school sand boxes. If your city has any parks with sand boxes or elaborate play equipment. Try there. Especially if it is wood chips or sand. Kids burry toys, Nannies and parents often loose coins while playing with their toddlers... I just checked a sand box at an elementry school (Kindergarten play lot) and found a few assorted coins. One was an French 5 Centimes coin. Also, a couple of coins were Wheat Pennies. I found a little charm from a girl's neclace or bracelet. It was a cheap, painted, alloy and I found some zipper pulls from childrens coats. With play areas, or any place for that matters, think of places where people are active, jumping around, hanging upside down.... Swings are good as there is a point, at the top of an arch, where peoples pockets are inverted and objects can fall out. I usually find most of my items about 8' out from the center lines of the swings. Also, horizontal bars or laddars where kids might hang upside down. Objects, money etc., fall from pockets at this time. Athletic fields are good also. Look along the sidelines, under bleachers etc. Where spectators might gather and loose items or where players pile up their belongings. Volleyball Pits are good as jewelry and coins can fall off or from pockets while someone jumps to hit the ball. My best find, a 14K White Gold ring with about 8 channel set diamonds, was found in a volleyball pit. I picture a girl going for the ball and her ring flying off when she hit it. The ring was in the middle of the pit in the sand. When going to a beach or park, look for places where people might go through their pockets getting keys out for their cars. Usually there is an area, between the park and parking lots where people walk to get to their cars. Follow that path. As keys are pulled from pockets, other items tend to fall on the ground, into the sand or whatever.
Look for swimming areas or beaches where people might sunbathe and loose jewelry or money as they remove their coverups... or where jewelry might fall off while wading in the water. Parks where there are festivals. Hopefully you can find some stuff, right after an event, that is on top of the ground.
I would be reluctant to suggest anyplace that requires digging in hard soil. It can be hard digging in the ground right now. Some areas have been really dry and are hard digging. It can be discouraging if you work hard and find junk or worse nothing. Also, Unless your son learns how to properly dig in and replace grass you can do more damage to a lawn or park. Maybe, in a wooded area where there isn't grass. Digging into bare soil would be OK. Get a trowel or other small hand tools. Learn to pinpoint an item so you don't have to dig as big of a hole. Don't go out there with a shovel and just start digging. Especially if it is dry near you, try to stay away from vegitation where it might be difficult to get enough water to recover.
I could go on and on, and there are many areas that would be good. Again though. To build up some encouragement or confidence, I would start with sandy areas. The beaches, play lots, etc. Also pick areas that see a lot of activity. Places where people get sit around sunbathing or picnicing... or places where they are active or inverted like athletic fields or play lots with swings and horizontal bars.
Good Luck, and Happy Hunting.
we will let you know of any "riches" we dig up. Thanks for all the helpI have another ?? it deals with detecting stone, and different rocks. I will see wherer I need to post the ?? at so we can get some more great help and advice. 
