Doggy Haircut?

If your dog gets too hot outside in the summer yes I would shave him. I have a Cocker and we have to shave him once a month summer and winter.
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i don't believe in shaving dogs except for medical reasons.
if he's to hot, get him a kiddie pool or let him play in the sprinkler.
make sure he has plenty of shade and water.
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For the safety of the dog in heat, I would. I have a purebred Maltese, and we trim him once every month during late Spring and Summer. He doesn't go out much, but when we don't he starts panting, and drinking water continually.
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I am assuming your dog is kept in full coat. Don't shave him as it can ruin the texture of his coat.

My mum had yorkies when i was little, but because she had less tme for them with having a baby, they were kept in 'puppy cuts', where the hair is still long enough for them to luke 'cute and fluffy' but nowhere near full length.

Here is a pic of puppycut http://www.all-about-paws.com/graphics/yorkie.jpg

Now compare this to full coat http://www.brolea.com/images/Shaun.jpg

Big Difference!!

(I used yorkie pictures as, from the description, your dog looks more yorkie than maltese, so it will sort of give you an idea of how yuor dig might look)
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Mum used to have yorkies