Hi and welcome to YT. Giardia can leave scarring in the intestinal tract which can lead to long-term digestive issues. There is a wonderful pet nutritionist, Susan Davis, who has worked with many of our pups with outstanding results. (
Pet Health Pet Nutritionist Holistic Pet)
Please do not use Science Diet. There are some great foods out there. If the vet is saying it is a digestibility diet, then you need foods that are easily digestible - most basic kibbles will not fit this bill. With digestive issues, the best thing you can do is get him on a daily probiotic. If he is having trouble digesting his food, then digestive enzymes may be needed. He may need other things down the road, but you can't go wrong with a consultation with Susan regarding his diet.