K.D.
Ann,
I am not sure about its availability to non-residents, but the Hollywood Park community has a nice open-air, covered pavilion with permanent picnic tables. It is associated with the community pool, and there is also an older clubhouse with available folding tables and chairs, a kitchen area and indoor bathrooms. Perhaps you know someone in the neighborhood who can reserve it for you.
I encourage you to view this event as a "family/friend party" that HAPPENS to coincide with your son's first birthday. Don't make him the focal point of attention. He is too young to "process" an event of this magnitude. Being the center of attention of 50 people can be exhausting and overwhelming for young children. So be prepared to get him out of the hoopla - isolated in a quieter indoor area on a blankie/pallet or in a travel crib, or better yet, have someone available to take him home.
The 'rule of thumb' for birthdays is to invite one "guest" for every year of your child's age. So for your SON's enjoyment, getting together another couple with a baby near his age (or 2-3 guests at most) is most appropriate. Parties of this nature - small, and gradually scaling up in the number of attending children as your child grows older - will make him feel more special. This provides room to host larger, thereby more "special" parties for teens. More focused events while he is very young will also contribute to BETTER memories of "his birthdays" when your older son reflects upon them.
Happy day,
K.