thank you everyone. I've always thought that they are only blowing bublles to make bubble nests when they are happy. But apparently they also blow bubbles when they are sick.

Here is a quote from www.bettatalk.com/answer81.htm that I find really helpfull

Blowing bubbles as a sign of happy healthy bettas:
happy healthy bettas blow bubbles: In the case of a happy betta, the bubbles are a bubble nest. The male blows a bubble nest because he is (well, let's just say it like it is) horny. Mr. Betta is dreaming of a nice damsel and wants to raise a little family, so he blows a bubble nest.
a happy healthy betta will usually be active in between bubble blowing. He will eat. If he hangs at the surface, it will usually be right under the nest (in a nest guarding position). Or it will be because he is actually sucking up air to build more bubbles (so you can see him taking a gulp of air, then going to the nest and blowing a nice bubble to add to his collection).
Blowing bubbles as a sign of sick bettas:
sick bettas blow bubbles: In the case of a sick betta, the bubbles are not 'happy-let's-have-a-spawn' bubbles. To be honest, I am not sure exactly why sick bettas sometimes create bubbles when they exhale at the surface, but they do.
a sick betta will not be active. He may not eat. He will hang at the surface with clamped fins, not move around much at all. The easiest way to tell a sick betta blowing bubbles apart from a healthy betta blowing a nest is by observing the bubbles themselves. If they are spare and scattered here and there (as in, one here and one there) and if they are large, they usually are sick betta bubbles. If they are smaller, in a large cluster, especially in a corner or under a leave, then they are a nest and they are happy betta bubbles.
I'm pretty sure Max's bubbles show that he is a happy betta
He is quite lively and finish his food within seconds, but don't worry I won't overfeed him.

His bubbles are concentrated in one corner and it gets bigger everytime I look at it. Will be sad to see it go when I do the water change but I'm sure he'll build another one in no time.