I have a 3-year-old and we visit frequently. I think that for most lines it's not not feasible to join up at the end anyway never mind if it's rude or not. I know my child and I know how long he can wait. I suggest starting out waiting in lines that are no longer than 10-15 minutes to see how your child does. If you download the
Disneyland app to your phone it tells you the wait times for all the attractions (however, it an be wrong, so definitely check the sign at the front of the attraction where you enter the line). I usually bring a snack or something special for longer lines. Your child may or may not like all the rides, so definitely let him lead when it comes to doing things, and start off with some gentle/easy rides or rides that you know he loves the teaming (for instance in Carsland).
Feed your child frequently (real food as much as you can rather than sugar and junky snacks). A well-fed toddler at Disneyland is far less prone to meltdowns. We pack a ton of food and snacks (sandwiches, fruit, veggies, crackers, applesauce, string cheese, etc) and also attempt to get at least one really solid meal with protein in during the day (grilled chicken and rice, grilled chicken skewers, bean and cheese burritos, even a hotdog). I am a firm believer in enough food (and the right kind of food) being one of the keys to a pleasant day with a toddler at DL.
If your boy is recently pottytrained and not totally reliable you might consider Pullups for the day. My friend's daughter had an accident in one of the boats on Pirates when we got backed up and had to wait a long time to exit the ride. They were both soaked. On that note, bring an extra shirt, pants and socks for your boy (and maybe a shirt for you). It never fails that we need a wardrobe change at some point due to food or something.
Respect your kid's mood. If he is cranky, exhausted, melting down or just needs downtime or a nap, skip the rides, parades and shows and find someplace quiet to chill for a while (indoors if it's hot). Also if it's hot, there are two splash pads in DCA (one is in Bugsland and one is near the pier--this one is only open in summer). A change of clothes comes in handy for this too. They must wear shoes in the splash pads so some water shoes or flip flops are good.
Also encourage him to drink lots of fluids. My boy definitely needs me to remind him or he will get dehydrated. He is just having too much fun to remember to drink enough water.
I hope you have an amazing visit. We have the best time together at Disneyland! It is really magical to go with your young child.