My son of 3 years old, has bottle tooth decay. Is there any risk of other infections spreading because of it? What else should we do except of brushing

Discuss with Pedo... Yes, teeth with decay can become infected and spread to any other part of the body. This would be very serious and needs to be avoided at all costs. The teeth should be repaired if possible, or extracted if necessary. Have a discussion with a pedodontist what would be best the best treatment for all teeth and how to prevent further dental problems in general.
Decay repaired. Your child's baby teeth are necessary for eating, smiling, and holding position for the adult teeth that come later. If you ignore the decay pain and infection will occur long before they are lost naturally. Your child requires dental procedures performed by a general or pediatric dentist.
See a dentist. If your son has decaying teeth, there is always a risk of an abscess and infection spreading through his bloodstream. It is rare, but can sometimes be very serious. Please see your dentist asap to have him evaluated.