Because your student said "dear" already, I don't recommend "hello" afterward. I'd prefer if your student chose between "dear" and "hello". It's not terrible to use both, but to make it sound less awkward, I'd pick between the two and then continue with "it's been a while" or "long time no see". Your student will be understood even if he/she sent this to someone who speaks English as their first language, but choice of words make the letter/message even better and more interesting! ^^ It'd sound better if "I'm missing you" was switched with "I've been missing you" or "I miss you". Here's examples: "Hello Mark, Long time no see. I've been missing you." "Dear Mark, It's been a while. I miss you." If your student wanted, he/she could've said: "I miss you because it's been a while" or "It's been a while, so I've been missing you" to spice up their message. Nonetheless, your students message isn't bad! :)

It is sincerity, That is enough :)

Not bad.