Itching in anus. I have told all things.

My son is 3 years old. Weight 14 kg born in 35 weeks. He has been continue itching in his anus for long time . For relief , he put pen or pencil to rub his anus or he find any sharp thing like table corner and then he sits and rub. Whenever he sit on my leg , he sits same try to move left and right. He is little bit constipated , he pass stool daily generaly but once or two time in a week , he poop alternate day. Poop thickness and length like bamboo PC's of 12-15 cm with 2 cm diameter . We generally give him deworm medicine like albendazole. Also he wet the mattress in night while sleeping. One or two time daily. What is the reason behind it. How to deal with it. Also he is allergic always  . Having cough strep throat ,stiff nose. Does it relate to itching . I have told all things.