Can you tell me is 145 lbs 5''9 too thin for a 17 year old male?

Large range: There is a large range of weights appropriate for a 5-9 adolescent. Some of it depends on his genetics (i.e. What the parents look like) and some depends on physical activity (is he a marathon runner?) if he is still growing, it will vary further. His growth chart can be interpreted by his pcp, who will be able to give you the hand-tailored answer for your son.