My 20 mth old has terrible pain ping his poo. Any natural remedies?

He apparently is constipated but pes stools every day or two. It causes him terrible pain to p large, bulky motions. He has to push so hard, I worry about his blood pressure at that point. The poo often breaks off and gets stuck in his causing even more distress. It culminates in slight tearing and bleeding of his which is obviously very painful and sore. It is normal colour and fairly soft - not dark small pebbles like you'd expect for constipation. GP's advise senna but when I was giving it regularly I think it was making matters worse as he was ping 'smears' of poo in his nappy all day long. I also think it could of been giving him tummy aches. He has a healthy varied diet and although dislikes most fruit and fruit juice, he eats lots of veg and drinks copious amounts of water. Any suggestions of the cause and/or natural remedies greatly appreciated.