For the guy that is ready to use a toy for this, I recommend using one by yourself first. This will allow you to get use to the sensations and feel of the toy. When you are ready to let your partner handle the wheel make sure you communicate with them how to use it. This includes how to insert it, how far in and how much pressure to apply.
Basically there are two types of toys that can be used. Those made specifically for Prostate massage like the Aneros/Nexus models and toys like dildo’s and vibes.
This is an example of toy that is inserted and moved “Hands free”
Models from Aneros and Nexus are of the hands free variety. You insert them and use your Kegals/Pelvic muscles to move them in and out. They typically have an extra arm on the base for perineum stimulation. It is generally accepted that these types of toys take time to master, mostly because the stimulation is light and you must learn not only how to make them move but also you have to learn to recognize the stimulation they give. To give you an idea, I have been using a couple of Aneros massagers for over a year and have yet to fully master them. I admit though, I just like to get off and I have not really put the time and effort into mastering them. I will recommend them to any guy though; nothing will let you learn to work your pelvic floor muscles quite like an Aneros Prostate massager.