Hi Aysa!
The decision on which language to teach, whether or not to teach several languages, and how to do this first is entirely subjective to you and your family - while your child is at that very young age, you have the opportunity to teach him multiple languages. The only key to doing this is to be consistent in your usage of the language (so your child has a contextual environment on which to use what he has learned) and to be CONSISTENT in your lessons in teaching him and the USAGE of the language in relating to persons, objects and environments.
I personally grew up in a household where we spoke at least 3 languages, and one provincial dialect (which is sort of a whole language in itself) and I do not remember having any difficulty with language learning. My nieces and the young ones on our family are currently learning multiple languages at home, where we use different languages interchangeably with no issues. 
So this will lead me to believe that there is certainly no harm to teaching your child different languages at a young age - in fact, I think this has contributed to fluency in the languages we learned, since we used them in context very often.
I think it may also be helpful if the people who told you that teaching a child more than one language at a young age may “damage his learning” can provide you with concrete evidence that this true. It may or may not, depending on circumstance, and the manner at how the child is taught the language.
Since your child is young, I personally believe that you have a great opportunity to teach your child, since it is during these years when a child is able to learn the most with little effort. And as long as the way the child is taught is filled with fun, and there is consistency in the lessons (this means that if you intend to teach a child a new language, you must also take care to use it and practice it with him), I think that your child will be able to learn with no issues at all.
But of course, don’t just take my word for it. :happy: I am sure other parents here in the forum will be more than happy to help you by sharing their own experiences in teaching their child multiple languages.
Here are some of the threads in the forum which discuss this specific topic:
Helping each other in teaching children multiple languages
http://forum.brillkids.com/teaching-your-child-signing-speaking-foreign-languages/helping-each-other-in-teaching-children-multiple-languages/
Bilingual Education
http://forum.brillkids.com/general-discussion-b5/bilingual-education/
Teaching multiple languages you don’t know
http://forum.brillkids.com/general-discussion-b5/teaching-multiple-languages-you-don’t-know/
Polyglots
http://forum.brillkids.com/teaching-your-child-signing-speaking-foreign-languages/polyglots/
Teaching multiple languages
http://forum.brillkids.com/teaching-your-child-signing-speaking-foreign-languages/teaching-multiple-languages/
How did you teach your child to speak a foreign language?
http://forum.brillkids.com/general-discussion-b5/how-did-you-teach-your-child-to-speak-a-foreign-language/
I really do hope this information helps you on your way to learning more about teaching languages, and please - feel free to share with us your opinion on it!