But he wants to get his VISA first before booking tickets or hotels.
It's advisable to make provisional bookings/ not charged to his credit card/ not confirmed, paid-for bookings.... and include these in the TRV application.
Do you think, he will get visitor visa without showing any concrete travel plans?
No.
The idea is to prove he has a well-defined purpose of visit/he has thought out in detail about what he'd like to do.... with detailed expense estimates + he must prove he has the finances to pay for this plan.
Is invitation letter a mandatory thing for Visitor Visa application?
It is not mandatory. That said, it could help your friend's visa application to some extent.
You must also provide a copy of your passport/immigration status + bank statement/NOA (your financial documents are NOT mandatory. You can provide them only if you are comfortable sharing it with your friend).
Can I as a friend provide invitation letter? Though he might be staying with me in Toronto only for 2-3 days when he is visiting and rest 10 days will be around Vancouver and Montreal in hotels, so I wonder what can I write in invitation letter for him!
Your invitation letter would state that you will support his visit for the 2-3 days he is visiting Toronto. For example, would you provide accommodation and show him around?
For the Vancouver + Montreal visits, he must include provisional bookings for flights and hotel reservations (none of these must be confirmed or paid for).
He has enough proofs to show ties like job, parents, properties, home loan, car loan, IT returns, Employment Letter etc. but could his being 36-37 and single be cause of visa rejection?
Age and being single does not increase the chances of a TRV refusal. Just last week a young teenager from Bosnia was approved. He is a student, traveling on his own without his parents, financially dependent on his mom, etc.
Your friend must demonstrate he has a strong purpose of visit (even if it is for tourism) + he has a good financial situation in his home country + no bad loans (just saying this) + properties owned by him (not by his parents/relatives) + he has previously visited other visa-required countries for short visits
