There is nothing much that you can do about your son being lazy. That is something that he will have to take care of on his own. If, however, there is any connection between your son becoming fat and his laziness, then it is an issue that you will need to tackle separately.
Right now, let's tackle the issue of his black back neck. The skin around our neck is made up of very fine folds that we cannot see. This makes it a perfect place for sweat and dust to accumulate. We also usually neglect washing our necks properly. And since, as you say, your son is lazy, chances are he must not be cleaning that area at all. Being fat, he must also be sweating a lot. Sweat contains uric acid and if left unattended, works on the skin by darkening it. Apart from that, if not washed properly, stale sweat can also cause rashes.
You can take care of both these problems by patiently and regularly applying these remedies on his darkened skin. First, let's take care of the rash. To heal and sanitize the rash, apply a thin paste of baking soda on the rashes. After that, apply a mixture of rose water and yogurt to his rashes. This will help to cool down his skin and take away any remaining itchiness. You can also apply a mixture sandalwood and rose water for the same results. Even plain milk cream that is chilled will help to heal the rash. Once the rashes are healed, you can start working on his darkened neck.
For this, the best and the easiest thing to do is to apply aloe vera gel on the affected area. This is something that you will be able to do everyday without any effort. For better results, apply it at before he goes to sleep. Alternatively, you can apply raw potato juice to his neck as it has natural bleaching agents. If you want to make the skin of his neck smooth along with lightening the color, try applying a cream made up of equal parts of almond paste, honey, milk powder, olive oil, and lemon juice on his neck. After applying it, see that he leaves it on for at least 30 minutes. Try and apply this paste on his neck at least once a week, if not more. Apart from all this, make sure that he washes his neck thoroughly everyday.
answered by G M