ADD "friend" = annoying.?

I’ve moved to a decent place, with cheap rent. Now I see why the rent is so cheap…it’s because I have to share with a mentally challenged boy (ADD)

He wants to go EVERYWHERE I go. If I drive to the gas station, he wants to ‘come for a drive’. If I drive to my girlfriend’s house, he wants to “come for a drive”. The supermarket. The car wash. To drop friends off or pick them up.

I’ve tried explaining to him (numerous times, quite politely) that he can’t ALWAYS come, but as he is, well, mentally challenged, he doesn’t understand. My girlfriend does not think much of him at all..and even said to me “does he EVER leave people alone?” He actually begs and begs to come.

Another issue is that most the time, he smells, barely washes, and just doesn’t shut up…he talks the most absolute BS to try and impress others (usually me)..

How can I explain, simply, to him that people sometimes need their own space?