Hey there,
My DD had really bad constipation around the same age and we ended up taking her to hospital where she was given a suppositry and me given advice.
Limit apples, give more pears.
try cooking prunes and mix with other fruit.
Plenty of water.
Change formula.. we used karicare..still go.. but she drinks heaps of water now... i also gave her very weak cammomile tea (u may want to look this up- i was brought up on it) and it helped. These days, spinach helps alot, a spinach sauce.
Goodluck, hope it goes well. Chemist may be useful too.