My suggestion is too look for commonality with his episodes with crying.
Wife and I found that both boys are sensitive to light, loud noise (especially noise) and crowds ... the youngest can have a whole mood/behavior change if he is overstimulated becuase of constant loud noises or being in a group of loud and rambunctious kids. Removing him from situations like that has cut down the amount of episodes.
If you deduce that it is a call for attention, judge if it is warranted, if not (ie being spoiled), then either tell him to change his mood or he can play by himself in his room alone, since no one likes to be around a grumpy gus. Be prepared to follow through with this threat, if he does not correct his behavior.
If he is actually very sad, talking is always the best medicine.
