God bless you. 何以唔說 God blesses you?

是否因為God 是要特別尊重 -- No I won't say that's why we don't use 'blesses', although it is true that because of respect we use captial letter for 'God' and 'Him'.
'God bless you' originated from 'May God bless you'. In here, 'May' is a model verb which doesn't change its form to add '-es' like other verbs would, as you'd know already. That's why everyone says '[may] God bless you' as they have dropped 'may' from sentence.
Hope that helps.

2008-05-31 14:05:11 補充：
And also, if a senetence contains a model verb, the other verb in the sentence stay the same (ie. in its infinitive form)
eg. He goes to the shops. / May he GO to the shop? / He may GO to the shop.

因為God是一個尊稱