What Phil said is best for approaching something/someone in a normal manner. But if you're up to no good or if you simply want to surprise someone, you might sneak up on him. 😈
~He sneaked up on the cat.
*You might hear 'snuck up' on the cat, but it's not accepted as correct in come circles. However, it is in the dictionary.
~Sometimes I sneak up on my husband just to see him jump. 😉
Sneak up (on somebody/something) phrasal verb
to get very near someone before they notice you
sneak up on: <em>Don’t sneak up on me like that!</em>
(Be) Up to no good-informal phrase
doing or planning something bad, illegal, or immoral