I guess I would first try to determine whether thing is just am "I'm a boy and I can aim my pee into a bottle" thing or something else.
It could (probably unlikely) be a sign of OCD. Howard Hughes used to keep his urine and feces in bottles and he was über OCD. OCD often emerges after a major life event/change. Does that apply? Does he hoard things? Refuse to throw things away, even if they are valueless? Collect many, many things? Do anything else in a patterned, ritualistic way? If not, he's just a normal boy fascinated with his pee-pee!