Is he actually having suicidal thoughts (e.g. wanting to kill die), or are the intrusive thoughts related to suicide? There's a big difference - the first indicates depression with anxiety, and the second indicates OCD.
If it is the latter, I've copied and pasted some info for you;
'Suicide obsessions are a specific category of suicidal thoughts that are unique to individuals with OCD. If you’ve read my previous posts on harm obsessions, you might conceptualize suicide obsessions as aggressive obsessions directed inward. Suicide obsessions involve repetitive, unwanted thoughts of suicide that cause severe distress. In contrast to other types of suicidal thoughts, suicide obsessions are symptoms of OCD that do not reflect a “true” intention to kill or harm oneself but rather reflect a repetitive thought loop that gets stuck.
Suicide obsessions can occur spontaneously, seemingly out of nowhere, or may be triggered by unpleasant (or even pleasant!) activities. These repeated, unwanted thoughts about death, suicide, or self-harm may occur many times throughout the day.'
'Suicide Obsessions & Self-Harm Obsessions
Here are some common examples of suicide and self-harm obsessions.
Fear of committing suicide in the absence of a desire to do so (e.g., fear of slitting your wrists, hanging yourself, or overdosing on medications).
Fear of becoming depressed and then feeling compelled to commit suicide.
Fear of jumping off of a building or another high place.
Fear of stepping out into oncoming traffic and being hit by a car.
Fear of jumping in front of an oncoming train or subway.
Fear of sticking your head in a hot oven.
Fear of putting your hand in a blender and turning it on.
Fear of grabbing ahold of a hot pan or stove burner.
Fear of putting your arm down the garbage disposal and turning it on.
Fear of driving off bridges or intentionally crashing head-on into other vehicles.
Fear of intentionally disfiguring your face with a curling iron.
Fear of intentionally crushing your car or yourself under the garage door.
Fear of losing control and intentionally blinding yourself.
Fear of mutilating yourself in other ways listed above.'
If the above does not apply, ignore me, but thought it was worth putting out there. x