I would say short trip training is a good start to booster readiness. His maturity level is a big part of whether or not he is ready for the step right now. Can you trust him not to unbuckle himself? Does he know what could happen if he isn't sitting still and staying in the right place? If you do start training I would recommend a high back booster off of the IIHS Best Bet list. But keep the Nautilus in the main vehicle (in harness mode) and use that whenever driving on the highway or taking longer trips.
When selecting one it my be a good idea to get one that also turns into a no back booster for longevity and has Best Bet ratings in both categories such as the Evenflo RightFit, Graco AFFIX or Graco Turbo Booster, Harmony Dreamtime or Harmony V6.
The Graco AFFIX has a nice feature for booster training, it uses the LATCH system in your vehicle to hold the booster in place when not occupied and makes it sturdier and easier for the new booster rider to climb in.
We are currently using a Harmony Dreamtime when a cousin occasionally rides with us. She is 5 and 45 pounds and still harnessed in her main vehicle. It was a cheaper option for us for the few times we have to use it. We like it just fine but beware if your kiddo doesn't have long legs it will be hard to keep him from slouching to bend his knees over the end since it has a very deep seat, which for some people is needed.
And even in Wisconsin, where it is illegal to Booster until after 4 years AND 40 pounds, we still see 30 pound kindergarteners in Graco Highback boosters because Graco says its OK! On the other side of the law, there are also 3 year olds in boosters because of the 30 pound manufacturer lower limits, which is even scarier to me!