Surely there's more for him --and you, and your son-- to do on weekends other than birthday parties and the park? Is there a bigger issue here? Does you husband PLAY with your son at all on weekends, rolling a ball to him in the yard, getting down on the floor and playing pretend games or helping him set up his little toy train set or zooming cars down the ramps of the Fisher-Price toddler garage set?
In other words -- is dad engaging his son at all, or engaging him plenty and the issue is just these outings? If dad's playing with your son, and really being part of his life, you're doing OK even if dad doesn't want to do parties and parks. But if your husband is just not really interacting one on one (without you there) with your son, it may be time for dad to learn to play with a 2-year-old and also to think of your son as someone to involve in his OWN fun.
What is your husband doing with this down time at home? Vegging out in front of the TV? Does he have any interests or hobbies? If he's into building stuff, your son can "build" next to him using a kids' plastic building set (little plastic chunky hammers etc., look for them in toy stores and resale shops). If dad's into the lawn, your son can, with your help pointing things out, pick up sticks from the grass before dad mows (and dad should be out there encouraging him to pick up sticks too!). If dad's into messing around in the garage with the car, well, a 2-year-old can sit on his lap in the stationary car and wipe, wipe, wipe the nice round steering wheel with a cloth while dad laughs and encourages him....If your husband says, "That would just slow me down, I want to get X done," remind him: All too soon your son will be old enough that he won't have much interest in sitting on dad's lap, toddling behind dad in the yard, watching dad zoom those plastic cars on the floor.
The idea is, your husband doesn't need to do this activity or always go to the party in order to interact with his son. He needs to start thinking of how to integrate his son into his daily life. He should want to spend time with his son doing anything or nothing on the weekends. It sounds like possibly your husband just doesn't yet see your son as a person who can do things alongside him -- he sees your son as the baby, and the baby stuff doesn't interest him. Two is too young to engage in a lot of ways, but not too young for dad to start spending time with him and playing. And please don't let your husband (if he's like this; he may not be) consider watching TV together to be spending time together. It's not. It's two people individually vegging out , not interacting. Just a thought.