My question is
I have a 5 month old male chihuahua he started loseing weight I can hardly get him to eat
He is very active.
I am very worried about this weight he is losing

What does your Vet say about his weight and eating habits vs what the Vet would recommend?

I agree with JKT, any drastic change like not eating and losing weight should always be looked at by a vet. Although many dogs go through a rebellious stage and slow their eating habits in their you teen-later teen age years, they normally don't lose drastic amounts of weight.

Is there any other symptoms? Diarrhea, vomiting, lethargy and is he still drinking plenty of water? Is he up to date with vaccines and flea and worm treatment?