Young children often have a limited attention span. Parents should not get anxious about this but rather should encourage their child's interest and attention without forcing the child to concentrate for too long. Enforced learning can be counterproductive if the child's attention span is limited. It is important to keep your son engaged and interested.
Products such as VTech's Thomas and Friends Laptop or Bobs Alphabet Much Truck will keep him stimulated so he can experience the rewards of learning and achieving with his favourite characters. This will, over time, lengthen his attention span and enable him to concentrate more.