Hello again, thank you for the history on your fellow, and I can understand your concern for him.
His inability to effectively take in air is definitely a concern, and because things have suddenly changed for the worse for him in a short period of time I do think this is something that should be looked into further.
Snub nosed breeds do have breathing challenges due to their conformation, called brachycephalic syndrome. These issues include an overlong soft palate, stenotic (small, closed in) nares, and parts of the larynx that evert out when breathing called everted laryngeal saccules. But when we see a sudden change as you have that means we have other complicating factors.
That can include a reaction to respiratory irritants (cigarette smoke, pollens, air fresheners), viral, bacterial, or fungal infection, dental disease affecting tooth roots which cause nasal inflammation, nasal polyps or a tumor.
Dogs that don't have their brachycephalic syndrome problems corrected surgically can suffer from exercise intolerance and easily suffer from heatstroke because they cannot breathe properly. Long term they can suffer from tracheal collapse and secondary heart disease from their struggle to exchange oxygen.
You should plan on adjusting his activity, but a veterinary visit to look for complicating factors and treating them as we are able can go a long way to making him more comfortable and improving his quality of life.
Please let me know if you have any further questions.