There are a couple of possible skin conditions here, which a doctor should probably examine. The first issue is the itching - this is possibly because of some kind of a rash, but there are numerous red itchy rashes and numerous causes for each rash.
When it comes to a thorough bath and complete skin care, the neck is often the most neglected zone but curiously, this area is also one of the moistest and most sweat-prone areas of the body. By not thoroughly scrubbing the area during a bath, bacterial and fungal colonies can take root, and once these start to attack the skin, a red itchy rash starts. To eliminate this as a possible reason, ask your grandson to use a body scrubber when bathing, and ensure that it is used over every part of his body. Using a body scrubber will eliminate any loose, dead skin that is stuck on the surface of the body. Usually, this in itself should solve the problem of itchiness.
You did mention that your grandson was slightly overweight, and this can be part of the reason for the itching. If your grandson is excessively overweight, and tends to sit or stand in a slouched position, the folds of skin in the neck area can overlap. This is then a perfect location for a fungus or bacteria to take hold since the region is constantly warm and moist. Your grandson needs to lose weight to solve this problem, and in the meanwhile, keeping this area dry would help. However, if the problem is too severe then a trip to a dermatologist would be a good idea, for oral and topical medication prescriptions and treatment, as well as some more skin care tips.
As for the dark skin on the back of the neck, this seems to simply be the result of a lot of time spent outdoors and exposed to the sun. Most probably, a case of increased melanogenesis has occurred, where the body is producing too much melanin as a response to the continuous exposure of a part of the body to ultraviolet radiation. In simple terms, this is known as a sun tan, and is only a temporary darkening of the skin. Ask your grandson to wear collared shirts more often, so that the back of his neck is covered, and is not constantly exposed to sunlight. Whenever he wears a collarless shirt, he should apply an adequate amount of sunscreen on his neck. This is in any case a good practice – sunscreen should be applied on any exposed skin.