no reason why you cant do this, except possibly if you are going AI as there would be a problem in that they would get classified as children for alcohol purposes and this could trigger under occupancy charges
I think you need to be sure about
what, if anything, the boyfriend is paying
getting written permission from his parents to cover him going and you having authority to make decisions in an emergency
the need to have the discussion now about what would happen if they split up before or during the holiday and the financial issues which could arise
are his parents happy about the rooming arrangements

Dependent on the country you are planning on holidaying in you might have problems in putting 2 under 18s of the opposite sex in a room on their own? Social attitudes elsewhere are not necessarily as liberal as they are here in the UK
but there is also the legal context given that they are both minors ie under 18 and you night find that when you produce everybodies' passports for registration that they insist on each sharing with an adult over 18. Who actually sleeps in each bed is up to you but you might need to be very discreet about that?

If you were going self catering then there is no problem about booking a child as an adult indeed that has been the sensible thing to do for years. Hotels as both Jim and SM have said are different. Was not aware that child prices applied to 16/17 year olds but yes other countries are far stricter about sharing of rooms, you need also to consider the legal position in some countries.
Regarding the age of the boyfriend you will have to obtain written permission from his parents/guardian.
Somewhere on HT there is a template letter for this. You might not consider it important but in the event of something going wrong it is then you will need it. You must also ensure that he has his own insurance and at that age it covers for adventurous activities. In theory should you not be able to produce the letter at the airport you could be refused boarding or entry at your destination.

Having trawled through HT have found this. Copy and paste as required.

The child must have their own insurance and the insurance company need to be aware they are not travelling with their parents.

Please also look here.

https://www.gov.uk/permission-take-child-abroad

I/We,.................................................................. ...........,
declare that I/we am/are the lawful Guardian/s of
....................................................., male/female, born
................................., at.(birth place)........................who
has a UK passport numbered ................................., which was issued
on .................................. at.....................................

My/Our child, .................................................., has my/our
consent to travel
with.............................................................of
(address)............................................,who has a UK passport
numbered .............................., which was issued on
.................................. at ........................

My/Our child is traveling to ..............................................
from the........ ......until .........,........ and will be staying
at..............................................

My/Our child will be leaving on ........................., and returning on
July ..........................

In the event that our child requires emergency medical treatment and we cannot
be reached,
.......................................................... is authorized to
consent to medical treatment in our stead. Any questions regarding this
document may be addressed to us at:

PARENTS ADDRESS

Signed............................................Witnessed
by............................................

Home Telephone: ..............................................
Work Telephone: ...............................................
Mobile Telephone: .............................................

This letter needs both your details and those of the child and their
parents.....think this would satisfy passport control and cover for emergency
treatment etc.....