No, it doesn't fade at all that quickly. Immunity generally wanes starting at about 10 years after the most recent vaccination. Having had a vaccination only 6 months ago, and the fact that the incident involved 2 small splinters, and the rarity of tetanus infection, I would not be overly concerned about this event.
I'm not sure where you live or what the immunization schedule is like, but in the US, babies start the DTaP as infants - at 2 months of age - and would have had 4 doses by age 18 months. If you are running behind schedule, please do keep in contact with your son's doctor to ensure that he gets properly caught up.