My aunt on my father’s side has five sons, but no daughters.

This cousin is her fifthyoungest son.

You could use fifth son but if she has five sons and he is the fifth, he is the youngest and it would be more normal to describe him as such.

I think thismy cousin’s son is excellengreat.

possessive 'my' would be better than 'this'. 'great', or 'fantastic', or 'amazing'. 'Excellent' used this way is more slang than regular usage.

He was born in 1994, and is the only college student among all themy aunt's grandsons of my aunt.

normal usage: my aunt's grandsons not the grandsons of my aunt.

I have met him a couple of times.

perfect case is better here. Also couple OF times.

I have always attached great importance to education, and I hope that the next generation of my family canwill study hard, but they seldom of them goes following my wishes.

'will' as you want them to do it. 'can' is have the ability to. 'seldom' is an adverb of frequency. It is not used for small numbers of people / objects so 'seldom of them' does not make sense. 'goes following' is not correct English, simple present is best here as it is a general wish not specific to a particular time.

I had mentioned my cousin’s son in my last post.

My aunt on my father’s side has five sons, but no daughter.

I think this cousin’s sonhe is excellent.

I have always attached great importance to education, and I hope that the next generation of my family can study hard, but seldomunfortunately very few of them goes following my wishes.