3 yr old trouble pooping!!! HELP!!

my son will be 3 on May 30th and he has the hardest time going poop. he strains and cries every time and it's just so hard to watch. he could sit on the toliet for 30 to 40 mins and still not go and we have resorted in using lacitives. it's just terrible that he is in so much pain and begging for "no poop" he gets anal fissures when he finally does go and clogs the toliet every time they are so big. we have tried upping his fiber and limiting dairy but he is such a picky eater it's hard for us to make him eat other foods. is there a drink mix maybe with fiber? I don't know anymore. I need suggestions on what to do. he has been to the doc and that's what she told us to do with fiber and dairy he has a 3yr appt June 15th and if nothing changes I will insist on a scan of his bowels just to be sure nothing is off. PLEASE HELP!