The best thing to do at this age is help him enjoy learning (and school - by making it fun), discover interests, and build social and personal skills. Young children put far less attention into activities they don't particularly like, have fairly short attention spans in general, and can be very active. If he is struggling in an area, work with him on it, but don't push him too hard or for too long.
Are their any museums, exhibits, or other institutions in your area that offer interactive science and technology experiences?
As to addressing deficits, you really need more specific information from your son's teacher about exactly what he is having trouble with. Ask lots of questions.
- What exactly is he having trouble with?
- What is he good at?
- What does he like to do?
- Does he have trouble getting started, or does he lose interest after a much shorter length of time than expected?
My wife works with preschoolers and kindergartners, and I have observed classes with children this age (my middle one is halfway through his first year of kindergarten). "On track" is a very general term for this age group, and change or learning often happens in fits. At 5, your son could also be on the young side, which makes a much bigger difference at this point. His overall progress may not be very well demonstrated by his ability to sit still and complete work (young or not).
That being said, there are some things he should generally be able to do by this point, which will be necessary to learn basic skills needed in primary school. Where he is in learning these skills is something you and his teachers should discuss as much as you can.
There is also an adjustment to be made to the demands of more structured learning, and this could be an issue as well. How much and what type of preschool did he attend?