You can use the itemalign key to change the alignment of the \sym text.
By default there is no space between the \sym text and the content for them item but you can use the distance key to set a value which is inserted between them.
You can also get rid of your manual font changes when you use the symstyle key to change the from from regular to bold.
\starttext
\startitemize[width=3.5em,symstyle=bold,itemalign=flushright,distance=.5em]
\sym{foo} Lorem ipsum dolor sit amet
\sym{foobar} Lorem ipsum dolor sit amet
\stopitemize
\stoptext
