It is fairly common problem after spinal surgery
compounded by analgesic causing worsening of constipation
Following things will help,
1) increase water intake to 3 -4 liters a day
2) Increase dietary fiber
intake with fruits, salads.
3) Can give him liquid paraffin
15 - 20 ml thrice a day
4) Can apply local lignocaine jelly before pooping.
5) Still having constipation then medications like laxative, enema or suppositories may be necessary consulting a physician.
Hope this answers your query.
Please do reply in case of any further questions.
Wish you a speedy recovery.