what you are doing is great - follow his cues on activities too - so if he likes to "walk" but only for a little while, then follow that lead (if it is maximum of 5 minutes, etc then you know when to cut it short before he gets upset).
when olivia was that age and got overtired and overstimulated so easily i let her sit and look at the trees blowing outside, gave her stuffed (non-noisy) toys to handle (texture, biting them, cooing, etc - good quiet interaction), she loved to sit on my lap while i sang nursery rhymes or she watched her sister (always loved to watch people and the shadows on the wall....)
i was advised by a nurse here that she was not missing anything by doing this - at that age the most important things are still food, sleep and love. parents of older generations never had the time to worry about "development" and "milestones" and all their kids got there in the end.
for tummy time i do think this is importamt (and thankfully Olivia liked it) - if it is not a favourite of your lo - do it in small blocks of time (start with 1-2 minutes) and build up from there aiming to get to more than 5-10 min per session several times a day over the next month. it is important for them to get that time because with babies now sleeping on their back they do not get the extra practice for holding head up, strengthening upper body, etc