Your seatbelt will have a broken internal spring to it's ratchet mech,not a DIY type repair.
I would fit a s/h belt,what you fit is your choice.
To fit either new or s/h you will need to access the reel by removing the rear seat side bolster(one 10mm bolt at it's lower end)then slide it upwards.Unbolt the reel(one 17mm bolt,I think),unbolt the upper mounting on the pillar(17mm,but you need to pull the plastic cover off first

)and the lower 17mm bolt.Take note of where and how spacers and washers are fitted on all mounting points,as these allow fittings to swivel as needed.
Of course,you could always mount the child seat at the other side of the car.....

Youth is wasted on the young.