Yes. In fact I advise transitioning kittens onto Junior food (or creating Junior by mixing kitten with adult food) at about 4mo and getting them onto adult food at 6mo. By this stage they have done most of their growing. The "feed till 12mo" on the back of kitten food packets pisses me off. IMO this is a big part of the reason that the majority of cats I see for their 1 year vaxes are already overweight (in fact we see quite a few 5-6mo kittens in for neuter that are already overweight) and then they are downright fat, or even obese, by 2. Partly because the owners have never seen them at a healthy adult weight and don't even know what it's supposed to look like. So yeah, change over now. Just buy a bag of adult food when there's a few days' worth of kitten food left in the bag, open the new bag and alter the ratios over about 5-7 days. That should do it.
Yeah ProPlan adult should be fine. There aren't really many shit foods any more- the standard of even the most basic dog and cat food has improved immensely over the past 20 years. See how he gets on. Read a bit about wet vs dry diets..I don't think dry food is the Great Satan or anything, but it is quite carby and can keep weight on them more than wet food (counter intuitively...wet food looks "richer" but isn't). Cats don't have a high thirst drive and on dry diets they often don't drink as much as they should to compensate for the lack of moisture in the food. If you feed both, don't mix them in one bowl- it makes the dry food just go crusty and manky after a bit. (Think nice fresh nachos vs cruddy nachos that have been sitting out for a while with congealed cheese and tortilla chips goung soggy).
Whatever food in whatever form you use:
a) don't let him get fat. Cats get fat underneath their belly first, way before they get chubby across their back. Watch out for that. Neutered, adult, mainly indoor cats have a calorie requirement equal to the square root of feck all.
b) encourage water intake by any and all means possible
And that's it, you're golden. If you're feeling fancy you can give him a wee sprinkle of probiotics. They do good things.
Oh yeah...don't ignore vomiting or hairballs. Cats can develop food allergies and because they often present as "merely" as bit of harmless seeming hairball puking (which has been normalised by TV/comedy films), some cats end up staying on food they have become allergic to for years. Short term- fine, though I wonder how many have secretly got sore tummies poor buggers. Long term- inflammatory bowel disease and guts like cardboard, in the end. Not good.