Hi there and welcome to BW!
30 minute naps are often OT (overtired) so you might want to check if his A time is a bit too long for him. Sorry, can't link as on my phone but if you look in the FAQ you should find something on average A times. There are also tips for sleep training with a toddler on the go! With my LO2, I had to accept that I wouldn't be able to focus so much on sleep training, so I would APOP (accidental parenting on purpose) the first nap by getting her up if she woke early in the nap and into a sling or buggy to continue sleeping. LO1 and I could then go out or play at home. But I also tried to get their afternoon naps to coincide so I could sleep train with LO2 while LO1 was asleep. And I also grabbed every chance at weekends and so on.
Hope that helps, lots of mamas here have had the same difficulty but it is possible, so take heart!