Help with anal retention

My toddler seems to be retaining his poop and we haven't started potty training so he's not afraid of the potty. He's retaining in the nappy. I see him pushing 5 times a day but he's resisting it. He has been constipated before and has done some pretty big poos but lately without any constipation he just resists. Today I gave him soup, chia and oatmeal yoghurt, lots of liquids. He was jumping on the trampoline (I didn't make him do any of these things or explained he had to, he had no idea why and was having fun) and at least 5 times he was having urges to push but he resisted and only small amount was coming out. I think he's scared because he has association with hair washing and bum washing and unfortunately he's very scared of hair washing (separate issue). I don't make him sit of a potty or anything, I just want him to overcome this and poop normally for the moment. He's 2.5. I'm scared, I don't know how to help him. I am positive, I never express any negative emotions towards poop, I'm always praising him and happy when he goes. He's definitely not ashamed, not afraid because something is detaching. I know he needs to overcome the hair washing fear and I'm very careful there but it could take months or years. His digestive system is important. Any similar stories? How long did the toddler resist?