It's great he and you were/are pretty regular. It is good in the long term to keep his routine/nap/sleep consistent.
How about just changing it to a "3" hour eat/play/nap routine now?
At this age, and at about 6 months old, he'll hit a growth spurt and developmental/milestone changes. He's going to start to get more gross motor skills, rolling over if not already, sitting up if not already, crawling and what not.
At any developmental change and growth spurt, this DOES temporarily put a hiccup in their usual routine. But it passes, and just keep regular about his routines. And their eating/feeding changes too. Due to growth spurt and developmental changes. Phases.
At this age, they still need to nap... usually morning and afternoon. A baby will still get sleepy/tired after about 2-3 hours of awake time. Even a bath is an "activity" for a baby.
For my kids when they were about this age, their "awake" time was pretty much a bath, using the floor gym, reading to them, putting on music and carrying them in my arms and moving around to the beat of the music, rolling around on the floor, using different toys with sounds and holding it in front of them and making "faces" and just talking out loud "to" them.
As for "playing" anything they do when awake is an "activity"...it does NOT always have to be literally "playing" with them... it is often good that a baby/child is ALSO "let" to play on their own... to discover on their own and experience things. Of course with supervision. That is how they learn. "Learning" and playing does not always have to be parent-led... it is great for it to be "child-led" too...let them show you things and make noises and discover... instead of always being told "how" or "what" to do. Then it is self-motivating and stimulating for them. They are sponges and will be entertained with anything pretty much. Even just watching you cook dinner for example, they are learning and it is interesting. Get a box of baby safe things... and let him topple it over, grab things, shake it around, etc. It's fun for them. They also just LOVE Kleenex boxes and just pulling out tissue after tissue. This is playing too. Or unrolling toilet paper. Go outside and let him explore. My kids when babies loved to lay down on a blanket outside on the yard and look up at the clouds and leaves in the trees... and especially found it "neat" when they heard an airplane flying overhead and then "saw" it, or saw birds or butterflies around.
Just make it age appropriate. Anything is an experience and a play adventure. It does NOT have to be all planned out and methodical. Spontaneous is great too. Anything is what they learn from and have fun with.