Many LOs have short naps or difficulty with day sleep from roughly 4-6 months. My DS was born 13 days over and acted like he was 2wks older than his age (if that makes sense) I saw short naps from 3.5-5.5 months as a result. I was quite amazed by this at the time, also exhausted, he'd been napping well until then but we had to do 4 or 5, 40 min naps per day! Often the day sleep gets better around 6 months though so although this may feel like a life time, it will pass.
I think I'd go for the CN in the morning and extend nap 2, every day so that there is a habit set, if you are planning to continue with swimming class long term. Although in the EASY routine we tend to see the CN last thing of the day until it is dropped really it can happen anywhere if it works out better for everyone. Due to our routine needs we did long nap then CN then long nap and eventually dropped the middle nap which was the CN.
If you don't choose to move the CN but continue as you are I think you will have to accept that you are disturbing his habit setting ability and so will need to continue to help him resettle that first nap every non-swimming day. If this is the case then yes I'd agree with not extending the other naps if it causes a problem fitting them in before BT but then you have the problem of needing a long nap 2 on swim day or he only gets 3 short naps...
It really is your choice but if it was me I'd change the whole routine so that every day is the same, babies learn sleep habits so I'd consider this the best option. I'd then also plan most of your out and about activities at roughly the same time as the swimming so that each day is relatively similar.
