And also he's 4 months and suffers from constipation... he poops once a day and has more wet diapers ...his poop is thicker like clay as he's on novalac ar digest and he pushes hard to make poo... what can i use to help him?

Pooping once daily with clay-like poop is normal for a 4-month old. Many babies push to have a bowel movement, unable to bear down properly. This is not necessarily a problem unless they are extremely uncomfortable. Try elevating their head when awake, massaging their tummy, bicycling the legs. Could add 15ml of non-honey based syrup to 1-2 bottles daily to help. See your doctor as needed.