Since my party lacked a proper Tank, I gave them an NPC construct with the lifespark subtype (he and his awakening are tied into the story of the campaign) and it worked quite well, they started teaching him about the world, about his newly found emotions and the magus started giving him melee training which resulted in the construct gaining levels of fighter. This started at level 4, now they have reached level 8 and have grown very attached to their metal friend. The problem is, without a CON score, his HP are advancing very slowly, a problem I hadn't considered in the beginning. Even with generously rounded 6 hp for each fighter level, the PCs are overtaking him one by one and there have been some very tight situations for him on the frontlines.
Now my players are searching for ways to buff him, so he survives, but since the normal +CON items don't work, I'm rather short on ideas.
They are willing to spend a considerable amount of money on their friend, so I would like to give them the option of actually succeeding.
How could this be done RAW and if there's no way, which houserules would you propose?
tl;dr: Need a way to raise the HP of a lifespark construct with fighter levels.