I have heard that its very common for around 5-6pm to be the grumpiest time in the day for babies... and I dont know the scientific reasoning behind this, but I have found it to be true, at least in our case anyway.
My son is a fair bit older than your bub though, but he has absolute tantrums at dinnertime (which is usually between 5-6) and will pretty much refuse to eat most things - even though he should be hungry! So I'm thinking I may adopt Christy's approach and feed him his main meal for lunch...
My thoughts is that maybe your DS is just very tired/cranky at that time of the day?? As often that is reason enough for them to scream and carry on and refuse their food. Because if it were a digestive issue I would assume that it would happen more than once per day, and at varying meal-times IYKWIM?

Not sure on the constipation issue though... I guess you could try a different formula and see how things go? Or make sure he is getting lots of pears in his diet, as that will soften things up a bit for him hopefully...