There are much less explicit rules for animals in 4e than 3.5, and this is on purpose. Generally, for our situation having a big tank to soak up some damage and help defend us when we absolutely need could prove critical since our group is small and struggles in some situations (another defender is the first/main thing we could use at this point imo). However, adding combatants will add work and slow down the game to some degree, so this should be balanced and kept in mind. We want to avoid the pitfalls and gain the benefits for everyone's fun.
The HUGE size is a significant obstacle for many situations, and so I am not sure if Bardolf the boar will be with the party all/most of the time. I'm copying the rules for squeezing here for easy reference. I'm just pointing out that in the future the size may bar us from wanting to keep Bardolf with us on some adventures. Similarly, though he is a mount, riding Bardolf opens the rider up to attacks from MANY more squares at once (any creature adjacent to the huge creature) and thus riding a huge creature is very dangerous for any save a defender (who generally speaking WANTS to be in the way and able to attack as many as possible at once).
SQUEEZE: MOVE ACTION
Smaller Space: A Large, Huge, or Gargantuan creature reduces its space by 1. For example, a Large creature that squeezes has a space of 1 (1 square) instead of 2 (4 squares). A Huge creature’s space changes from 3 (9 squares) to 2 (4 squares). When a Medium or smaller creature squeezes, the DM decides how narrow a space the creature can occupy. If an effect prevents a creature from leaving a square in order to squeeze, the creature cannot squeeze.
Half Speed: As part of the same move action, move up to half your speed.
–5 Penalty to Attack Rolls: You have a –5 penalty to attack rolls until you return to your normal space.
Grant Combat Advantage: You grant combat advantage to all enemies until you return to your normal space.
Provoke Opportunity Attacks: If squeezing causes any part of your space to leave a square adjacent to an enemy, that enemy can make an opportunity attack against you.
Ending a Squeeze: You can end a squeeze as a free action. You return to your normal space. You have to occupy a space that includes the space you occupied when you stopped squeezing.