Ds bowel difficulties

[Icelollycraving]

Ds suffers a great deal with pooing. He sometimes poos seemingly constantly. He does also get constipation. His tummy feels soft so not impacted. Been given lactulose several times now by gp.
He does eat well,lots of fruit & some veggies. He does love fish,eggs & dairy particularly. He drinks water & milk,as well as weak squash.
He really grunts & goes red for every poo. Today there have been maybe 5 slightly soiled nappies along with a firmer poo (sorry for tmi).
I've limited bread,dairy. None of it really makes a huge difference.
I was hoping someone else had encountered this & what works. He is 2.5.

Also it does smell pretty bad so as he is permanently soiling. He always sits on the hardest surface he can find when pooing,no idea if that is relevant.
He is generally aside from this a very healthy little boy.