His body slam needs counterplay in melee range because currently it has none or at least lower reward. Urgot treatment with short delay would do the trick or maybe Vi treatment: the longer he channels the more damage, CC and range he has or if they want to make his dash unique as the one of the few insta ones just make it apply reduced CC and/or reduced duration of it when hit in low range.
As for ability to stop dashes: it would most likely require for him to lose his hard CC on E, displacements stop dashes (Quinn E, Yasuo Tornado, Thresh Q and E, Blitzcrank Q, Janna R etc.). Getting rid of displacement from Grag's E lowers his skill ceiling as it needs to be taken into account in order to do geometry combo so I'm not really a fan of this.
As for damage they need to put more emphasis on making use of his charged Q: reduce damage of noncharged Q and increase it on the charged one. They could also remove or reduce slow on basic Q and make it apply on the charged one, it would reduce his already good sticking power and his safety quite a bit. Rest is quite needed in order to clear jungle at reasonable speed.