"God bless you" is in the subjunctive mood, because it's a wish. It's not slang/informal. Verbs in the subjunctive mood don't change with the subject. For example, one of the most common uses of the subjunctive mood is to talk about things that are not true, which is often used in conditionals. And for the verb "to be", the subjunctive mood is always "were". You don't say "if I was you", but rather "if I were you". Similarly, you would say "if he were you" and "if she were you", etc. The subjunctive mood is also used to talk about wishes. "God bless you" is a wish. It's like saying, "May God bless you." Regardless of the subject, the verb, "bless", stays the same. "God bless you" and "God blesses you" mean different things. The second one expresses a thing that is true, not a wish. You can read more about the subjunctive mood here: http://grammarist.com/grammar/subjunctive-mood/

Hi Tenir, It depends on what you're trying to say. If it's a greeting or a wish, then "God bless you" is fine. If it's a statement, like "Rain falls," then "God blesses you" is correct. Technically, if you are saying "I pray that God bless you" it's the subjunctive mood, so it doesn't take the -s you usually see with the third person. Otherwise, if you say "God blesses you every day," it's a statement, so you include the s at the end of the verb. Hope this helps!

we have to add s with verbs only for third person . example of he, she, it example : eat I eat you eat she/he/it eats only for present simple positive and negative - we don't use 'S' with verbs in question form - we don't use 'S' in past action also