There's a problem with your use case and the current tech--artificial locomotion in VR tends to cause people to experience motion sickness. It's similar to sea sickness in that some people don't feel affected at all and others can be full on vomiting within a relatively short time of exposure. It's a natural reaction to the dissonance between your eyes visually saying that your body is in motion but your inner ear saying your body is at rest. Skateboarding/rollerblading specifically are both pretty extreme sports in terms of movement, so any sims with current tech are liable to make him feel nauseous after any kind of serious exposure. Most current VR experiences have 1:1 movement in the real and virtual world to avoid this problem, and the ones that don't tend to limit artificial locomotion to slow forward movements to try and cut down on the effects. Actual skateboarding/rollerblading are going to be pretty risky for him to try and enjoy in VR. Odds are they're going to just make him feel sick to his stomach. (Also, as a former skateboarder--most of the skill in the sport is balance and footwork. None of the headsets are tracking your feet, so it would be pretty hard to get a realistic sim built for it.)

That being said, the best tech out there currently is the Vive IMO, with the Rift likely tied once its touch controllers ship this December. Both have 6 degrees of freedom when tracking you. But right now the Vive is the only system that officially supports tracking your movement within a few square meters, and has motion controllers supported. That means within a room, you can walk around in both real life and the game world simultaneously and reach out and interact with the virtual world. The presence you get from that kind of experience is impossible to describe. Once the Rift's touch controllers ship the two systems will likely be on par with each other.

The mobile headsets all have 3DoF tracking. That means that the rotation of your head is tracked, but not its position in 3D space--taking a step forward in real life won't also move you a step forward in the virtual world, but the direction in which you look will be 1:1. You don't have as immersive experiences on them because of that, but for experiences where you're a passive/seated observer you can still get a VR experience for a tiny fraction of the price of a Vive/Rift + VR capable PC. Their performance depends on the quality of your phone.

For your grandfather I'd actually recommend he try and get a demo of the Vive or Rift on the floor of a PC store. Microsoft and Micro Center stores were both giving demos of them when the Vive debuted. That way you could gauge how much he enjoyed the experience and see if it's something you want to invest in for him in general. Maybe pick up a cardboard and find 360* skateboarding videos on Youtube just for him to experience it, if you were going to buy anything blind--that would be a ~$20 investment, and for those sports specifically you probably aren't going to find anything better on the high end systems.