How to encourage exercise

My son is 10 years old, 5" 2' tall but now starting to gain a bit of weight. His exercise regime is limited to that which is provided at school plus 30 minutes swimming once per week and the occasional short and slow dog walk (2 miles max). He is higher functioning according to the diagnosis. I wondered what successes other parents have had. Has any body found boxing or pad work a success given how tough 3 minutes is?