Thank you very much for the answers.
He must surely build some muscle because he's really out of shape, I hardly manage to make some exercises with him. Now it's winter and it's very hard with all the snow and cold weather.
And I must teach him to lunge. He was used for carriage working by some gypsies. They often used hobbles at the front legs to limit movement so I was thinking that his narrow chest is a consequence of this practice. They use those things even for young horses. And besides of that, for them it is a normal practice to put at work a young horse at 1,5 - 2 years of age
He was found on the city streets in a poor shape. Anyway, it's hard to understand why he's such a kind horse when you think at his past.
Now he's used very rarely at riding, he's still a green horse and he can't understand leg aids.
And sorry for my possible mistakes, I'm not very good at english.