I solved my room over a garage problem early this year for right about $550 just blowing dense pack cellulose in the 5x20 ft kneewall, sidewalls ceiling and in the garage ceiling. Do make sure you are getting enough air thru your vents as well having enough return air ducts for the room. Adding supp heat would likely solve your problem but is pricey to install and then run all the time. Your DH from what it sounds like didnt insulate the kneewalls correctly buy just laying down the fiberglass. Fiberglass insulation wONT STOP cold air from going thru it at all. Are you using the kneewall area for storage? If he can still get in there have him spray foam using a foam gun on the kneewall/floor junction and make sure its sealed off. If you arent using the place for storage insulation should only go on the actual back of the kneewall; its pointless to put it on the floor in there and the slope. He is going to need to put more than the usual r13 batts on it too and cover the area with 2 inch think r10 foamboard to serve as a good air barrier and additional insulation for a total of at least r23. If the space isnt too deep you might think about filling it totally with cellulose as i did mine. My kneewall area was 5x20ft and had a~2ft space behind it. since i couldnt get in there i just filled it with 30 bags of cellulose. Difference was immediate. The other big thing i did was go and check out the garage ceiling and found only 2 inches of fiberglass there in a 12 inch space with it resting on the garage ceiling. This left all that space above for cold air to run around under my floor. I again drilled a series of 2 inch holes thru the sheetrock all along the rafters and used a fill tube to snake past the old fiberglass and filled the entire ceiling of the garage with cellulose--again about 30 bags. The sidewalls and the ceiling of the bonuse room i did like you read about on various sites.
So in the end i went from a room that we could almost never use in winter or summer to having my child sleep and play there comfortably all year long for a little over 500 bucks. Read the link below for how and where to airseal the kneewall as well as needed r values.

Here is a link that might be useful: room over a garage

You didnÂt mention whether the garage was attached or not which can change solving the issue. Assuming that it is attached, many times bonus rooms are an afterthought because they were finished out later due to construction costs or the desire to keep off the tax base until moving in.
Being an afterthought, typically the ducts are not sized properly and/or are placed properly to handle to room needs. In addition the current heat/ac system may not be large enough to account for the large bonus room added to the system.

The heating system and duct work should be checked first to see if it is capable of taking care of the room. If not , one option is to upsize, and you may wish to split the areas off with a warm air zone system, as bonus rooms tend to be removed from main house and have 3 sides, along with ceiling and floor in contact with unconditioned spaces.

Second choice woould be a dedicated system such as an additional furnace with AC if desired or a boiler, depending on budget and ease of access.

Proper insulation is key and should be the starting point before a good contractor calculates the room loads, checks the existing loads and decides if capacity is there or more needed.

How much do you use this room in the winter?

Where are you located?

How large is the room?

What kind of heat do you currently have? If you have forced hot air, you'll have problems adding hydronic heat.

There may be better, and far cheaper, solutions to the problem.

We would like to use the room more often in the winter than we do. It really is the kids playroom and their escape during the cold winter months. Right now during the cold months we plug in a small heater during the day only to take the edge off of the cold.

The room is approx 400sf, above an attached garage.

We currently have forced air that uses that hydro-system and of course, the flexduct going in to the bonus room is the furthest from the furnace. The bonus room also is under a split-roof (I think that is what you call it). Where the roof of the main house is higher than roof over the bonus room. Access to the ceiling area of the bonus room through the main attic is not possible since there is a wall where the lower roof drops down. Someone suggested we have some type of access built into the bonus room attic area anyway.

Just curious why the hydronic heat will not work with forced air since my sister who just finished their basement had hydronic heat installed and they have forced air on the upper 2 floors.

We are located in lower NY state.

Thx for your help.