Dear Mum, before I give you some practical suggestions, I would ask you to try to understand why your child doesn't want to fall asleep: perhaps he doesn't want to be away from you and from the fun activities that you do together because he doesn't see you much during the day, or he may perceive sleep as something strange, outside of his conscious control or … it might even be an attempt to stay up until you too go to sleep. Try to respond and in the meantime here are some tips to overcome his "resistance to sleep".
The first tip was inspired by your words: you don't need to "tire out" children in the evening because the more tired they are, the more irritable they are, they make an unnecessary fuss and don't want to go to sleep. Create a repetitive routine that accompanies the child each evening heading towards bedtime, doing everything calmly and without stress (for example, having a bath, brushing teeth, reading a story, talking about the day….) perhaps with some small variations: one evening you tell a fairy story, the next day it's Dad...
Before bedtime avoid games or activities that can create excitement and use gentle and repetitive music. Once put to bed, the child must stay there, and if he is afraid of falling asleep, stay nearby and reassure him. A soft doll might be useful (the transitional object) that can comfort him and help him with his separation. After the last goodnight kiss and before leaving him alone in his bedroom, always ask him if needs anything else.
During the day don't darken the room where he has his nap, while in the evening, have a dark room and turn on a soft light to differentiate day from night. Don't let him nap after 16.30/17.00 because this could interfere with his night sleep. Rest well….