how old is Jake? when they are young, it is easy for them to regress in their training or to sometimes confuse their commands.
I would keep the training sessions short--only 10-15 minutes at a time, and always be sure to end on a positive note-if you see that he is getting tired or distracted, do one last command correctly and then stop. you do not want to end the session frustrated or having him ignoring you.
Also, if he is having trouble mastering sit and down, I would not add in anything new until he is consistent and competent with those.
What command words are you using for each command? Be sure you are being consistent in your direction/command--are you using "sit" for sit and "down" for down? then be sure you are not saying "sit down" when you want him to sit--this will confuse him because to him you are asking for both, and he will hear the "down" last and probably do that. also, be sure vary your requests--if you always ask him to "sit" and then ask him to "down" he may be becoming conditioned to always go into the "down" after he is told to "sit"...
If he goes into the down when you ask him to sit, correct him and give him the command. For instance, if you say "sit" and he goes into the "down", place him in the sit position and tell him "sit", when he stays in the sit, praise and reward him. if he keeps going for the down instead, try only working on the sit command in a couple sessions, no "downs".
