Lemon and cranberry juice will help with urinary tract infections as a preventative in susceptible individuals but will not cure an active infection. Your dog's urine should really be checked for infection with a urinalysis.
Please bring a fresh urine sample to your vet for analysis. You may not even have to bring your dog in for an exam if the urine is positive for a simple urinary tract infection (an infection without crystals). The vet can simply give appropriate antibiotics.
If your dog has crystals in his urine, the crystals would need to be identified and treatment instituted. Crystals can be dangerous in male dogs because if they become large enough they can become lodged in the urethra and cause blockage (even in dogs, though more common in male cats). Most urinary tract infections do not involve crystals and can be successfully treated with antibiotics.
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