After how many days of constipation should I go to the hospital because it would be dangerous?

My 7 1/2 month old baby has been constipated for 4 days. He started purees at 5 months and DME at 6 months and it never happened to him to be constipated at this point. I've been giving him juice and prune puree for 2 days, he drinks water, eats fruit, I give him belly rubs and leg exercises but unlike usual, it doesn't work. He drinks less milk, eats much less since yesterday and has a very bad stomach. The pharmacist suggested me to give him laxaday, which I will start tomorrow morning.