Actually...The author does say a little of what Ali's reaction was...

"We both looked to Ali, but of course with him you could never tell. His face was impassive as ever, though something sober had melted into his eyes."

It's not much, granted. But let's look at the deffinitions of 'sober' and see if we can come up with how just this little bit of information could have a much larger meaning...

Sober-
1. Habitually abstemious in the use of alcoholic liquors or drugs; temperate.
2. Not intoxicated or affected by the use of drugs.
3. Plain or subdued: sober attire.
4. Devoid of frivolity, excess, exaggeration, or speculative imagination; straightforward: gave a sober assessment of the situation.
5. Marked by seriousness, gravity, or solemnity of conduct or character. See Synonyms at serious.
6. Marked by circumspection and self-restraint.

We can rule out 1 and 2, as this simple comment has nothing to do with alcohol...

We can also rule out 3 and 4, because the prior statement of 'his face was as impassive as ever' pretty much sums up those deffinitions.

Number 5 doesn't quite fit right...Because it's just in his eyes, that Amir comments on, becoming sober...So it's not a physical soberness...

It's purely an emotional soberness, which is what is discribed in deffinition number 6. So the question that arises then, is what is going on in Ali's head, that has him feeling circumspect and showing self-restraint about the gift of Baba to Hassan?

That little bit of a blurb shows that Ali isn't happy with the gift for an unknown reason, but also feels that he can't refuse the gift to his son either. That because it's coming from Baba, he has no choice but to except Baba's decision in this matter concerning Ali's son, Hassan...

You also get another tiny hint, as to why Ali would react this way...The leading up to Baba presenting his gift to Hassan...

The author talks about how in previous years, Ali has always known what Baba was getting Hassan. And that the boys were able to get the information from him, of what the gifts were...But this time, even Ali is kept unaware of the gift and has no explination to the boys of why he's been kept out of the loop this time around.

So what unwritten message is appearing in this gift giving? What exactly is going on in the relationship of Baba and Ali in regards to Ali's son, Hassan?

On one hand, in decisions of giving gifts of a frivolous nature (toys) Baba consults with Ali...But in decisions regarding Hassan's quality of life he does not consult Ali? And even though thier 'master and servant', Ali just hands over authority to Baba, in that regard...He's not happy about it, but something in his eyes 'sobers him up and restrains him' to the reality he has no say in the matter in regards to Hassan's gift.

So is it he feels that although he doesn't like the idea, that maybe Baba knows it'll make Hassan's life easier...Is it a class thing, where as a servant he feels he has no say if his master makes a decision about his or his son's life? Or is there something more to it and the author is unaware of the reason at this time?