Best Way to buy a second property

I want to buy a second property to rent out. Value approx 150K

I have a property worth 400K, 100K mortgage outstanding for 9 years. Current mortgage deal is 2.4% fixed for 2 years.

Q's:
1. Can I simply borrow more using my existing mortgage to buy the second property?
2. Can I have two mortgages at any one time?
3. Should I use a Buy to Let mortgage (interest rates - worse than my 2.4%). I have a guaranteed tenant (my son!)
4. Is any / all of the above legal?

Thanks
Chris