We had Ds christened when he was 11 months, which was last July. He wore a smart short sleeved shirt and pair of shorts as luckily the weather was great. BHS, do wedding stuff, so they have smart shirts and trousers I think at quite a good price. Or theres M & S. You don't say how old your Ds is?
Afterwards everyone went to the pub next door, where they laid on a simple buffet, I think for about 30 people = £100 - bargain! and we bought an iced fruit cake from M & S, which I stuck Ds' Christian names on with chocolate letters (from sainsburys) and some small marzipan animals (also sainburys' and it looked fabulous.
Getting back to the service, the vicar came to our house a few weeks beforehand along with godparents, to discuss what goes on (it was held during a normal Sunday service,) what will be said, so we would know what was going to happen. We also were allowed to choose all the hymns.
Maybe you could contact your vicar and discuss it with him?
With regards to guests, we invited who WE wanted and not because Great Aunt Fanny would be upset IYSWIM. (We did the same at our wedding - Well it was our day/party!)
I think the day is for your son and if you don't wont all the shannigans, then you don't have to. It's a bit like organising a wedding, you (supposedly) are in charge and can have it as big or small as you want.
Good Luck - Lets us know what you decide and how it goes.