my son, a two year old boy started attending preschool
Atopic dermatitis likely
I can understand your concern.
I have carefully reviewed all the images you have provided and, in my opinion, it seems to be atopic dermatitis. This is a chronic itchy inflammatory skin disease of unknown origin that usually starts at an early age.
Typical signs of atopic dermatitis are skin dryness, itching, thick, leathery patches of skin as well as a chronic and relapsing course of the condition.
There is no specific diagnostic test for this condition. However, a complete blood cell count is necessary to see for increased IgE levels which would confirm the diagnosis, as well as to exclude a deficiency in the immune system.
The best treatment for this condition in small children are moisturizers, such as Aquaphor, Petrolatum or Atopiclair. You can also use mild corticosteroid creams, mineral oils or tar based products. You can use them multiple times per day, whenever you see the child's skin getting dry.
When you give the child a bath, make sure the water is lukewarm and do not exceed 10 minutes. After the bath gently dry him with a towel, making sure the skin is not completely dry and then immediately apply the moisturizer. After application, wait a few minutes before getting him dressed, to make sure that the skin absorbs the moisturizer.
There is always a risk of an additional skin infection caused by scratching and if you notice that the child's skin is irritated, scabbing and oozing, please contact your doctor for an antibiotic prescription.
I hope this was helpful. Let me know if I can assist you further.