孩子更亲近他妈妈,怎么办?

That is totally normal. My recommendation:
这很正常。我的建议是：

Be chill. The woman (your wife; his mom) played a much bigger role than you did in his early life, so you are pretty much screwed if you expect an even playing field for the first few years.
冷静点。这位女性（你的妻子，他的妈妈）在他早年的生活中扮演的角色比你重要得多，所以如果你希望在最初的几年得到一样的爱，那你会很生气。

As a rule, you should pretty much let him be with his mother whenever he wants; that's the best way to build trust.
一般来说，只要他想，你就应该让他和他的妈妈在一起，这是建立信任的最佳方式。

When you are with him, do a really good job of taking care of him -- feeding him, changing him, putting him to sleep, etc. Get him to trust that you're competent and confident.
当你和他在一起的时候，你就要好好照顾他——给他喂饭、给他换尿布、哄他睡觉等等，让他相信你是个称职且自信的爸爸。

When you are with him alone, you should be able to distract him quickly from thinking about mom.
当你和他独处时，你应该让他能从对妈妈的想念中转移注意力。

It's normal for children to be more attached to one parent than the other at various times during their development.
在孩子的成长过程中的不同阶段，他们更亲近父母中的一方是很正常的事。

At times it will be for one parent, and later, without any rhyme or reason, it can switch to the other parent.
有时他们更喜欢父母中的一位，接着莫名其妙地，这份喜欢可能会转移到另一位身上。

As long as there's not something in your relationship with your son that might be causing the distance (ie, lack of time together, lack of affection, anything that might be effecting a strong bond forming), then don't worry, your time will come!
只要没有发生什么会疏远你们父子关系的事情（比如缺少共处的时间、没有感情，任何可能会影响你们之间形成牢固联系的事情），那就不用担心，你会有机会的！