I will examine the time of his disappearance. That would be retrodiction which requires the same techniques as prediction. I use both transits and solar arc (sa) directions in either retrodiction or prediction. Since we don't know his exact time of birth, we will probably not find anything conclusive -- only things suggestive. I will report his transits first.
I checked his birthchart for things that I consider suggestive of a tendency to suicide. I found nothing, but that is not conclusive since we don't have is angles since we don't have his birth time. However, I found that your brother is prone to risky behavior. He may have enjoyed danger, and so he is more prone to danger and intrigue than most. His natal Sun is at the midpoint of Mars/Pluto. He is powerful and can be dangerous. Venus-Pluto is at the Mars/Uranus midpoint. He can be too pushy with females. He will flirt with the boundaries in a way that can cause a female to take defensive measures. Mars squares the Sun/Uranus midpoint. He is accident or catastrophe prone. He is impatient and irritable. I have been focused on dangerous characteristics in his chart.
On June 7, 2021, there was a transiting opposition between Mars and Pluto that had his natal Pluto semi-square the midpoint. This is very dangerous and potentially deadly, but it does not suggest suicide. There was a transiting Mercury-Neptune square that placed Neptune in opposition to his natal Mars. This looks like a serious case of fraud or confusion. So far, it looks like a drug deal gone bad. His transiting Jupiter trined his natal Sun, so he was feeling optimistic, confident and open. He was not expecting trouble. This also tends to trend away from the suicide theory.
In solar arc, the sa.Ju-opp-Pl placed sa.Pluto sextile n.Sun with sa.Jupiter trine his Sun. He was not feeling suicidal according to this, but he might have been feeling driven. sa.Sun was sextile n.Mars and possibly n.Moon. He was eager and vigorous. sa.Mercury was conjunct n.Venus suggesting good humor and light-heartedness. sa.Uranus was square n.Venus, suggesting something happening with a female. There may have been a severance of a relationship, but it seems more likely that he was hoping for sexual excitement. sa.Sun/Mars midpoint was squaring n.Neptune. Again a drug connection.
sa.Saturn is squaring n.Mars (slightly wide) suggesting potential for death. Sa.Mars/Saturn midpoint conjuncts n.Uranus suggesting that death was unexpected and quick.