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I want to update this thread, just in case anyone is wondering.
The visa was approved eventually.
What we have done:

I have given him my bank statements with around CA$15k sitting there for 6 months.
I wrote him an invitation letter.
I prepared a list of expense and jotted down how much he would be paying.

He added a statement from a PayPal-like account stating about CA$4000 for the past 6 months.
He gave payslips of about CA$70k annual salary.
He added a letter from his employer saying he will return to the job.

Idk which exactly helped, but anyway it worked. Now we are happy.
 
I want to update this thread, just in case anyone is wondering.
The visa was approved eventually.
What we have done:

I have given him my bank statements with around CA$15k sitting there for 6 months.
I wrote him an invitation letter.
I prepared a list of expense and jotted down how much he would be paying.

He added a statement from a PayPal-like account stating about CA$4000 for the past 6 months.
He gave payslips of about CA$70k annual salary.
He added a letter from his employer saying he will return to the job.

Idk which exactly helped, but anyway it worked. Now we are happy.
Hi,Im applying for a similar visa and I would like to talk to you regarding this matter. Can I connect with you over email or any other social media?
 
Never lie to immigration officers. If it is the intention to one day apply for PR status for him then him visiting will help to cement that yours is an ongoing long term relationship and failing to mention the purpose of this visit could raise flags later. There is nothing wrong with him visiting his friend / girlfriend for a vacation. Additionally, Immigration officers can (and have) gone through people's phones / computers to see if the reason they are giving for visiting is genuine, on arrival. It would only need one email or text to show a different reason to make things difficult in the long run.

However, you must make sure he has proof that he will return to Hong Kong at the end of the visit, that he has ties and reasons to return. I would suggest he supplies (and carries) the following:

Return Flight Booked *proof of intent to return
Letter/email from employer confirming vacation time being used - and return date to work (or email to employer showing same) *reason to return
He should show his employment contract - showing his salary, to prove his employment is ongoing and he is solvent.
He could show his last 3 months' bank statements, with an explanation that he took a trip (and show proof of that - flights/hotels etc.), if finances are a concern
Receipt for rent / lease document of rental in Hong Kong *ties to return
Confirmation of any doctor / dental / other appointments in the month after his return (almost worth making an appt in advance for this reason) *reason to return
Invitations to any social events he has been invited to - such as a colleagues wedding or engagement party - after his vacation ends *reason to return
A letter of support from you as his friend/girlfriend - detailing he will stay with you for the duration of the vacation; you will cover accommodation and day to day living expenses should help, together with proof of sufficient funds to do so.

If you intend to travel in Canada whilst he is here - show the itinerary and any booked hotels etc., - so he can honestly state that he is coming to visit you and to travel within Canada.