My LO cant poop on his own

My 3 weeks old baby needs to use glycerin suppositories or i have to insert a qtip with vaseline in his little but to make him poop. Hes been like this for almost 2 weeks. His using similac alimentum hes always grunting and straining and crying because he wants to poop sobad but he cant at all even though he makes a grest effort to do it. Ive try massages but they dont work at all. The crazy thing is when he poops his poop is really watery so i do not understand why is it so hard to him to push it out. I already talked to his peditrician and im waiting for an appointment with his stomach doctor to check any food alergies or anything related to it. Had anyone have this issue before???