Asda also do a vapour bubble bath thats suitable from birth too. Just bathed both mine in it to try clear them out before bed x
Little and often is probably best when they’re poorly and for hydration best way is monitoring number of wet nappies, or if they’re excessively lethargic. You are honestly doing all the right things with the saline spray, steam in bathroom to humidify etc. if you’re worried about their hydration into tomorrow I’m sure (or would hope) your GP would be happy to chat things through with you or examine for signs of dehydration ☺️
As long as he’s still having plenty of wet nappies, don’t worry too much about not drinking enough. Just keep doing little and often, if he’s struggling to do it all in one go. Snuffle babe do a vapour rub that you can use from 3 months which may be worth a try. Currently going through the same with my little one! I tend to put it on before sleep as thats when she’s at her worse but might help with feeding for you 🤞🏻 x