OK, Mia-- I just looked at the chart you posted.
I can't say for sure how your source came up with its prediction, but here's what I found in your son's chart.
So Mars does rule the cusp of your son's 8th house, with Pluto in Sagittarius in the 8th, and Mars opposed Pluto. Then Sagittarius is on the cusp of the 9th house, which rules overseas travel. Mars rules things made of iron and steel, warfare, and the like. Possibly your source looked at the 8th as the house of death, but did an even simpler calculation, in that some astrologers see the house following the house in question as proceeding out of it, or as indicating what follows from it.
In other words, Scorpio, traditionally ruled by Mars is on the cusp of the traditional 8th house of death, as the traditional ruler of Scorpio. So this suggests phenomena related to Mars. The 9th and Sagittarius follow from the 8th, which indicates foreign travel.
Maybe your source neglected to mention that the 9th house also rules places associated with higher education, religion, great literature, and other mind-expanding activities.
I think this is the rationale behind the death prediction you found.
As the others have noted, every mother is bound to worry about a child enlisting in the military, but that plus a simplistic interpretation of the 8th house are not grounds for trying to prevent him, or to worry yourself sick.
Astrology does contain the good, the bad, and the ugly; and the bad astrology should have no place in our lives.