Bir oğlum var. The "u" drops there. There´s a rule about dropping vowels.

 Do you know, I knew that  - I did it right the first time - then thought twice about it and changed it!  But thank you so much for your correction meltem.

 It´s ok hehe. Usually the one that comes to your mind the first is correct. Probably you had heard it before.

 It should be: Bu kötü değil.

Bunu = accusative, but in this sentence you used it as the nominatve form, because it is the subject of the sentence.

Bu güzel, ama bunu beğenmedim.

This is nice (this = subject of the sentence), but I didn´t like this one (this one = object of the sentence, accusative)

Not to be picky or something, just thought you would like to learn

 I think that Caliptrix explained this once saying that the number implies the plural so that adding "lar/ler" is redundant.  That really helped me remember.  Also, Deli_Kizin thanks for the correction.