Should Lax-A-Day be used in a child only during periods of constipation or on a regular basis?

My 2 year old son has been suffering from constipation for 1 month (he has never been constipated before). The episodes of constipation occur twice a week and last about 2 days. He seems to suffer a lot (cries, says boo-boo, screams in pain and repeats 'poo' all the time). The pharmacist has recommended 'lax-a-day' but I'm wondering if I should only give it to him when the episodes occur or on a daily basis? I'm afraid he'll become dependent on it. We are also trying to get him to drink a lot of water and give him a lot of fruits, vegetables and other things...it's not very easy because sometimes he refuses to cooperate! Thank you in advance for your advice!

Yes he consumes a lot, he just wants to drink milk or water. But mostly milk. I will try to decrease his milk intake and increase his water intake.