你这个句子完全正确，这就是语法上所谓的错综时间虚拟条件句（即主句与从句的时间不一致）。

▲从句（过去）+主句（现在）

If the doctor had come in time, Amy would still be alive. 如果医生及时赶到艾米现在还会活着。

You would know what was going on if you’d listened. 你若注意听了，就会知道发生什么事了。

If it had rained last night, the ground would be wet now. 要是昨晚下过雨的话，现在地面就会是湿的。

You would be much better now if you had taken my advice. 假若你当时听我的话，你现在就会好多了。

If you'd listened to me, you wouldn't be in such trouble now. 如果你听了我的话，你现在也不会有这样的麻烦了。

If I had spoken to him yesterday, I should know what to do now. 假若我昨天对他说了，现在我就知道该怎么办了。

▲从句（现在）+主句（过去）

If I was a man they would have given me the job.  我要是男的，这份工作就准给我了。

If you were in better health, we'd have let you go with us. 你要是身体好一点，我们就让你和我们一道去了。

If you had only the least thought for others you would not have spoken out in that way. 你若稍为他人着想，必不会那样说话。

▲你的句子：

If he were alive now, he might have grown into a handsome young person. 要是他现在还活着的话，他会已长成英俊的小伙子。

从句指现在情况，即指现在还活着；主句指过去，即到现在已经长成英俊的小伙了(”长成“的过程在过去)。如果用 might grow…，也不错，但意思不同，它表示将来，即将来会长成英俊的小伙（但现在还不是英俊小伙）。