It is typical of the bulbous glands to come
up suddenly, quite large and hard and then to disappear soon
after. A lump on the base of the penis which seems quite scary, then suddenly disappears, to come up again another time is almost always bulbous glands.
The fact that he's been neutered....right, how long ago was he neutered? If fairly recently, then there is plenty of testosterone still in his system and may be for a few weeks yet -to cause sexual stimulation like that.