My suggestion is buy him a sunshirt/rash guard for the summertime - it won't be THAT huge on him, but will cover him to help keep him warm in the water now and use a swim diaper. You can buy the matching swimtrunks now for the summer too, but they may just slide off him. So perhaps skip using them on this trip, but use them in the summer to protect him from the sun.
I think JC Penney already has there summer suits out. Old Navy does not. Lands End has WONDERFUL swimwear that I really trust to last all summer, not stretch too much and really protect in the sun - which I view as ESSENTIAL for babies especially. The are more expensive tho.
And as for packing, less is more, esp for one night!
You can ask the hotel to keep your bag or just leave the biggest bag IN the car. We bring in a big "swim bag" with our pool essentials - swim diapers, suits for everyone and a coverup/big sweatshirt for the baby. When we are getting ready to leave, my husband checks us into the hotel, brings up the bag while I slowly get the baby ready and start heading towards the room.
That way HE takes the luggage up and is waiting there for us with the pack n play set up too!
Have fun!