Does he always complain about everything?
Is he genuinely miserable about things or is it just the way he acts?
Firstly if it's just a one off, then I don't think he behaved appallingly over the presents. He was disappointed looking in comparison, yes, he shouldn't, but it can put a feeling of being worth less. Look at the adults on here who see a smaller present and equate it with lack of worth to the giver.
One year dd1 had fewer but more expensive presents at Christmas and I told her beforehand this was the case, so they knew it was equivalent. They agreed that a mobile phone was worth a lot more than soft toys and building blocks and were happy, but I think their initial reaction could have been disappointment looking at the equivalent sizes if I hadn't warned them.
Sometimes a bit of managing expectations "if that's what you want for Christmas then you'll be getting fewer presents because they're expensive".
And a bit of thinking how to deal with it-if he had notably fewer presents would he have felt better if say you'd got a couple of things he'd have had anyway (eg new pens for school, socks, etc) to make the number of presents equal.
Sometimes you can't predict what they'll love/how they'll react. I have a policy that if they see something a sibling got that they wish they had one, they tell me quietly, and I'll note it down for a possible birthday present.
Normally they say they wanted one in the heat of the moment, then they come to me later and say actually they don't want one. But it diffuses that jealous moment, and when they think about it in cold light of dawn they realise they didn't actually want it that much. It also means that they make less fuss about it at the time, which means said sibling is much more likely to share it. 
But sometimes I think something might cause jealousy. Ds asked for an Alexa this year for his room. I expected his siblings to feel this was firstly a rather big present compared to normal and secondly that it wasn't fair because they wanted one. Both have looked at it and said "why do you want one?"
Equally well, the other two looked at dd2's orange twirl and hoped they had one too (they didn't, they had equivalents). They reached a compromise. Dd2 gave them one finger to share and they each gave her one of their fingers. So dd2 had extra chocolate, and they got to taste some of hers. Actually I thought dd1's chocolate (which was bigger) might be the envied one, but, as I said you can't predict it.
But I'm going to also say my dbro was negative about everything. Presents, days out, activities, food etc.
From my prospective as an older sister it felt like it was done on purpose to get things. We'd have days which went like this:
Dm takes us somewhere as a treat. I say, "thank you, that was great". He says, "I only went because you insisted on me going and I didn't enjoy it."
So dm thinks "oh it wasn't a treat for him" so buys him an extra cake to eat at home, because I'd had a treat.
He says (while eating it) "I don't really like those cakes but I suppose I have to eat it."
So dm spends the afternoon playing a game with him to make up.
He says "I only did it because I know you wanted to play it"... etc.
We had many days like this.
Very frustrating as an older sibling.
Now what happened one summer holidays was my parents decided they would find one thing that he enjoyed.
So they took him to all sorts of things, just him, ate out (we almost never ate out) and us older ones stayed at home, even when it was things that we'd have enjoyed.
Each time he came home and said.
"I only went because you made me," and the parents sighed and started planning something else they thought he'd like.
Then dm asked him if he'd like to go to an activity day on something he enjoyed. He gave his normal answer of:
"I suppose I have to go if you want to go."
I remember being particularly miffed because I'd asked to go to a similar one a few years previously and told it was too expensive and too far. This was more expensive and further.
The day came and they didn't go.
About midday he came through and asked when they were leaving as "he supposed he'd have to go."
Dm said that something else had come up and as he hadn't seemed that keen, she'd decided they wouldn't go.
He was furious.
Dm realised from that that the "miserable" act was literally that. He was enjoying them, but just making them unenjoyable for everyone else around.
I did resist saying "I told you so".
Now as an adult I'm not sure whether he was (as I thought) manipulating it to get as much as possible out of it. Or whether he enjoyed moaning about it. Or whether it was a bit of a self preservation in hiding what he really did enjoy because it made him feel exposed.
I'm not sure he would be able to answer that himself.
He did struggle with positive emotions like saying "thank you" and saying that something was fun or he'd enjoyed it. I think it was made worse though by the clear expectation of my parents that he would act as though he'd had fun.