Against advice, when my infant son gets small bumps I often jump up to hold and comfort him while he bursts into tears. Is it true that I shouldn't react with this behaviour?

I have to agree with everyone here - depends on age and how severe the bump is. For a newborn, I would definitely check in on every minor or major bump, for a child above 4 months, I would wait and observe from a distance and see if he/she can self soothe first. Being sensitive to the possibility of self-soothing is the beginning of believing in your baby. Recognize that challenges and stress are a natural and important part of a baby’s growth. The ability to manage stress and muscle through challenges builds self-esteem and self-confidence.