I am a Clinical psychologist in UK and Dr Olsen thought that I might be of help with your question.
For a child to be diagnosed as Autistic, he/she needs to meet certain criteria and present with impairments in social interaction and communication and demonstrate other repetitive or odd behaviors. I have attached a link for you to read the criteria in more details http://www.autreat.com/dsm4-autism
Also, for the diangosis of ADHD a child would need to show persistent difficulty in concentration and focus and be overly active in general, fidgety and showing some difficulty completing tasks.
What you are describing, this overexcitment could be common in some children but depending on the severity and frequency of it may indicate some difficulty in controlling his impulses.
I would suggest that you speak to the school and check if this behaviour is displayed there and on what occasions. Also read the criteria for Autism and ADHD. If you feel that he presents with some of these difficulties then you need to visit his GP and suggest a referral to your local Child Developement Service. Some boroughs/PCTs have their own separate service for children under 5-6 and other PCTs see all age children under CAMHS (child and adolescent mental health service). Your GP would know where to refer him if this is needed for further assessment.
This behaviour would need some monitoring and if you think that it becomes a problem then you would need to see a Psychologist to learn how to help him identify his feelings of being calm and being overexcited, learn how to apply boundaries for this behaviour while at the same time introduce a reward system and positive reinforcement.
I hope this helps
All the best.