Sigh...my 28 months daughter same. Before 2 years old, she pooed daily. But when she stopped drinking breastmilk, her pooing gets more and more difficult. She eats prunes, tons of veggie & fruits (cos she loves them), probiotics...Tried changing all kinds of milk powder but not improving. Now on diluted fresh milk(1part water to 2 parts fresh milk). So now she is able to poo 2 or 3 days once. Previously went to pediatrician and he told me to stop giving her any milk products. Didn't try that cos I still think there must be a better way...didn't want her to lose any nutrients while growing up.
She is also very scared of pooing. 1 month ago, she had fever and was very dehydrated. She didn't poo for 1 week. Tried suppository (glycine capsule that is pushed up the rectum) myself but didn't work! Had to go to the doctor the next day for edema. Thank God it worked. Since then, even more fearful of pooing.
But I think that probiotics worked. Helps to regulate her digestive system. I tried kid's kyodophilus (available at Nature Farm). Vanilla flavour. My daughter loves them. She says it's like candy. Supposedly give 1/2 to below 4 but she is taking 1. It is really expensive $40+ per bottle of 60 pills. But I found a US website selling it. Vitacost.com which sells US$7.03 (no need to pay US tax) for the same bottle and delivery to Singapore (or internationally) is $6.99 (for 5-10 business days).
You can try.