My brother-in-law is doing triathlons and is currently training for Ironman. He has a ton of energy, but is also very health conscious, and very disciplined. His wife (my sister) is supportive of him any way she can, whether it's doing some training with him, picking out recipes they can cook together, or just encouraging him. Maybe try asking him how he wants you to encourage him or support him. Maybe your way of being supportive right now isn't working for the both of you.