How effective are Dulcolax suppositories for constipation in Toddler?

my 2.5 year old son has chronic constipation since birth and has been taking lactlose to begin with but was started on Movicol about 4 weeks ago. for the past 6 days there has been no motion in spite of his daily dose of Movicol. we got him some microlax enemas this morning and tried them but no luck. i phoned our GP this evening and he told us to buy some Dulcolax suppositories in the morning and insert one of these. if they don't produce a motion within 2 hours we are to bring him to A&E to have him esed and X-rayed. have you ever used Dulcolax suppositories? how well do they work? i have used the tablets myself and they are very effective when taken orally but i don't know how well the suppositories work. will they work quickly and efficiently? please tell me your experience.