Can you bring your son with you and have someone care for him at the gym or close by? That way you could nurse just before you work out, and then just after, and you wouldn't interrupt your 2-3 hour pattern.
If that is not an option and you are going to be away for 4 hours, you have several options.
You could express some breastmilk and leave it for him. His care provider could feed it to him in an open cup, or a straw cup, or a sippy cup.
You could leave some solid food and water while you are away, and nurse just before you leave and right when you get back.
You could leave him without any food or milk - but if he nurses every 2-3 hours then he may be really hungry and upset by the time you get back.
Im going to suggest one more option....make sure you are home within 3 hours. within the next few months, his eating will (most likely ) pick up and he will be able to go longer without you.
Happy at Home Mama to DD 4/95 DS 4/98 and DS#2 8/10