You could enable SQLCMD mode:
:setvar myvar 10
declare @aid int = $(myvar);
select @aid
go
declare @aid int = $(myvar);
select @aid
Or simply:
:setvar myvar 10
select $(myvar)
go
select $(myvar)
This is an other approach:
declare @aid int = 10
create table #vars(aid int)
insert into #vars values(@aid)
select @aid
go
declare @aid int
select @aid = aid from #vars
select @aid
[Update] SQLCMD mode is not available in ReportingServices